setup_board: fix --regen_config bug.

https://gerrit.chromium.org/gerrit/#/c/38156 introduced a bug where
cross_get_version was moved to install_toolchain but was still
used in setup_board.

setup_board now saves whatever toolchain version is currently configured
and appends that information to the newly generated board config.

BUG=chromium-os:36865
TEST=remote trybot. manual - test case of existence/nonexistence of
board config file and existence/nonexistence of previous toolchain info.

Change-Id: I20e1794c12e7e60fdbbcd5a123ddc5c6bdf86b16
Reviewed-on: https://gerrit.chromium.org/gerrit/39132
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>

setup_board

@@ -389,6 +389,11 @@ cmds+=(
 )
 sudo_multi "${cmds[@]}"
 
+SAVED_VERSION=
+if [[ ${FLAGS_regen_configs} -eq ${FLAGS_TRUE} ]]; then
+  SAVED_VERSION=$(grep -s ^LIBC_VERSION= ${BOARD_SETUP} || true)
+fi
+
 sudo_clobber "${BOARD_SETUP}" <<EOF
 # Created by setup_board
 CHOST="${FLAGS_toolchain}"
@@ -397,7 +402,9 @@ BOARD_OVERLAY="${BOARD_OVERLAY_LIST}"
 MAKEOPTS="-j${NUM_JOBS}"
 PKG_CONFIG="pkg-config-${BOARD_VARIANT}"
 BOARD_USE="${BOARD_VARIANT}"
+${SAVED_VERSION}
 EOF
+
 if ! ${HOST_BOARD}; then
   sudo ln -sf "${CHROMIUMOS_CONFIG}/make.conf.${ARCH}-target" \
     "${BOARD_ETC}/make.conf"
@@ -508,12 +515,6 @@ else
       sys-kernel/linux-headers
 
     unset KERNEL_EMERGE_FLAGS
-  else
-    # Since we clobbered this file earlier, make sure the libc version
-    # is set correctly otherwise future upgrades will wrongly trigger.
-    sudo_append "${BOARD_SETUP}" <<EOF
-LIBC_VERSION="$(cross_get_version glibc)"
-EOF
   fi
 fi