diff --git a/bin/cros_make_image_bootable b/bin/cros_make_image_bootable
index 6fb60d1ac0..6d33cf1cfd 100755
--- a/bin/cros_make_image_bootable
+++ b/bin/cros_make_image_bootable
@@ -33,6 +33,7 @@ assert_inside_chroot
 
 # Load functions and constants for chromeos-install
 . /usr/lib/installer/chromeos-common.sh || exit 1
+. "${BUILD_LIBRARY_DIR}/toolchain_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/build_image_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/disk_layout_util.sh" || exit 1
 
diff --git a/build_image b/build_image
index 9800b25388..86f9b0a08c 100755
--- a/build_image
+++ b/build_image
@@ -71,6 +71,7 @@ check_gsutil_opts
 
 # N.B.  Ordering matters for some of the libraries below, because
 # some of the files contain initialization used by later files.
+. "${BUILD_LIBRARY_DIR}/toolchain_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/board_options.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/disk_layout_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/mount_gpt_util.sh" || exit 1
diff --git a/build_library/board_options.sh b/build_library/board_options.sh
index 6281018824..5a75022048 100644
--- a/build_library/board_options.sh
+++ b/build_library/board_options.sh
@@ -9,24 +9,7 @@ fi
 
 BOARD="${FLAGS_board}"
 BOARD_ROOT="/build/${BOARD}"
+ARCH=$(get_board_arch ${BOARD})
 
 # What cross-build are we targeting?
 . "${BOARD_ROOT}/etc/make.conf.board_setup"
-
-# Figure out ARCH from the given toolchain.
-# TODO(jrbarnette): There's a copy of this code in setup_board;
-# it should be shared.
-case "$(echo "${CHOST}" | awk -F'-' '{ print $1 }')" in
-  arm*)
-    ARCH="arm"
-    ;;
-  *86)
-    ARCH="x86"
-    ;;
-  *x86_64)
-    ARCH="amd64"
-    ;;
-  *)
-    error "Unable to determine ARCH from toolchain: ${CHOST}"
-    exit 1
-esac
diff --git a/build_library/toolchain_util.sh b/build_library/toolchain_util.sh
index 231f9b3ea8..2548330440 100644
--- a/build_library/toolchain_util.sh
+++ b/build_library/toolchain_util.sh
@@ -11,7 +11,8 @@ TOOLCHAIN_PKGS=(
 )
 
 # Portage arguments to enforce the toolchain to only use binpkgs.
-TOOLCHAIN_BINONLY=( "${TOOLCHAIN_PKGS[@]/#/--useoldpkg-atoms=}" )
+TOOLCHAIN_BINONLY=( "${TOOLCHAIN_PKGS[@]/#/--useoldpkg-atoms=}"
+                    "${TOOLCHAIN_PKGS[@]/#/--rebuild-exclude=}" )
 
 # Portage profile to use for building out the cross compiler's SYSROOT.
 # This is only used as an intermediate step to be able to use the cross
@@ -26,6 +27,30 @@ BOARD_CHOST["amd64-generic"]="x86_64-cros-linux-gnu"
 BOARD_PROFILE["amd64-generic"]="coreos:coreos/amd64/generic"
 BOARD_NAMES=( "${!BOARD_CHOST[@]}" )
 
+### Generic metadata fetching functions ###
+
+# map CHOST to portage ARCH, list came from crossdev
+# Usage: get_portage_arch chost
+get_portage_arch() {
+    case "$1" in
+        aarch64*)   echo arm;;
+        alpha*)     echo alpha;;
+        arm*)       echo arm;;
+        hppa*)      echo hppa;;
+        ia64*)      echo ia64;;
+        i?86*)      echo x86;;
+        m68*)       echo m68k;;
+        mips*)      echo mips;;
+        powerpc64*) echo ppc64;;
+        powerpc*)   echo ppc;;
+        sparc*)     echo sparc;;
+        s390*)      echo s390;;
+        sh*)        echo sh;;
+        x86_64*)    echo amd64;;
+        *)          die "Unknown CHOST '$1'";;
+    esac
+}
+
 get_board_list() {
     local IFS=$'\n\t '
     sort <<<"${BOARD_NAMES[*]}"
@@ -41,6 +66,15 @@ get_profile_list() {
     sort -u <<<"${BOARD_PROFILE[*]}"
 }
 
+# Usage: get_board_arch board [board...]
+get_board_arch() {
+    local board
+    for board in "$@"; do
+        get_portage_arch $(get_board_chost "${board}")
+    done
+}
+
+# Usage: get_board_chost board [board...]
 get_board_chost() {
     local board
     for board in "$@"; do
@@ -52,6 +86,7 @@ get_board_chost() {
     done
 }
 
+# Usage: get_board_profile board [board...]
 get_board_profile() {
     local board
     for board in "$@"; do
@@ -63,6 +98,7 @@ get_board_profile() {
     done
 }
 
+# Usage: get_cross_pkgs chost [chost2...]
 get_cross_pkgs() {
     local cross_chost native_pkg
     for cross_chost in "$@"; do
diff --git a/build_packages b/build_packages
index 4084ee3a4c..21234f69b3 100755
--- a/build_packages
+++ b/build_packages
@@ -126,6 +126,7 @@ fi
 "${SCRIPTS_DIR}"/setup_board --quiet --board=${FLAGS_board} "${UPDATE_ARGS[@]}"
 
 # set BOARD and BOARD_ROOT
+. "${BUILD_LIBRARY_DIR}/toolchain_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/board_options.sh" || exit 1
 
 # Setup all the emerge command/flags.
diff --git a/image_to_vm.sh b/image_to_vm.sh
index a617ea41e0..c6727987b1 100755
--- a/image_to_vm.sh
+++ b/image_to_vm.sh
@@ -12,6 +12,7 @@
 # Helper scripts should be run from the same location as this script.
 SCRIPT_ROOT=$(dirname "$(readlink -f "$0")")
 . "${SCRIPT_ROOT}/common.sh" || exit 1
+. "${BUILD_LIBRARY_DIR}/toolchain_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/disk_layout_util.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/build_common.sh" || exit 1
 . "${BUILD_LIBRARY_DIR}/build_image_util.sh" || exit 1