Add option to create a hybrid MBR

This allows booting on boards that require their bootloader be on
the first partition of a MBR formatted disk

BUG=chromium-os:32150
TEST=Build an image using the --hybrid_mbr_hack flag, verify that
the ESP is visible on a system with GPT support

Change-Id: I4b137ef672b9ed7327bd42ec0a260d82a8c9d470
Reviewed-on: https://gerrit.chromium.org/gerrit/26071
Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Ready: Liam McLoughlin <lmcloughlin@chromium.org>
Reviewed-by: Liam McLoughlin <lmcloughlin@chromium.org>

build_image

@@ -64,8 +64,8 @@ show_help_if_requested "$@"
 # not needed for the typical developer workflow.
 DEFINE_integer build_attempt 1 \
   "The build attempt for this image build."
-DEFINE_string version "" \
-  "Overrides version number in name to this version."
+DEFINE_boolean hybrid_mbr ${FLAGS_FALSE} \
+  "Creates a hybrid MBR rather than a protective one"
 DEFINE_integer jobs -1 \
   "How many packages to build in parallel at maximum."
 DEFINE_boolean replace ${FLAGS_FALSE} \
@@ -81,6 +81,8 @@ DEFINE_string verity_algorithm "sha1" \
   "Cryptographic hash algorithm used for kernel vboot. Default : sha1"
 DEFINE_string verity_salt "" \
   "Root filesystem salt. Default: randomly generated."
+DEFINE_string version "" \
+  "Overrides version number in name to this version."
 
 # Parse command line.
 FLAGS "$@" || exit 1

build_library/base_image_util.sh

@@ -245,4 +245,15 @@ create_base_image() {
                                                 ${image_name} \
                                                 ${USE_DEV_KEYS}
   fi
+
+  # Setup hybrid MBR if it was enabled
+  if [[ ${FLAGS_hybrid_mbr_hack} -eq ${FLAGS_TRUE} ]]; then
+    info "Creating hybrid MBR"
+    sudo sfdisk "${BUILD_DIR}/${image_name}" <<EOF
+unit: sectors
+
+disk1 : start=   $START_ESP, size=    $NUM_ESP_SECTORS, Id= c, bootable
+disk2 : start=   1, size=    1, Id= ee
+EOF
+  fi
 }