fix(disk_layout_util.sh): Fix changing build_image's disk layout.

Previously the code in base_image_util.sh properly handled the disk
layout command line flag but the spaghetti code later on calls a
function from disk_layout_util.sh which only returned 'base' resulting
in a bit of a mess if something other than 'base' is used. Sync up the
two code paths to avoid that...

build_library/base_image_util.sh

@@ -39,11 +39,9 @@ cleanup_mounts() {
 create_base_image() {
   local image_name=$1
   local rootfs_verification_enabled=$2
-  local image_type="base"
 
-  if [[ "${FLAGS_disk_layout}" != "default" ]]; then
-      image_type="${FLAGS_disk_layout}"
-  fi
+  get_disk_layout_type
+  local image_type="${DISK_LAYOUT_TYPE}"
 
   check_valid_layout "base"
   check_valid_layout ${image_type}

build_library/disk_layout_util.sh

@@ -112,6 +112,10 @@ check_valid_layout() {
 
 get_disk_layout_type() {
   DISK_LAYOUT_TYPE="base"
+  if [[ -n "${FLAGS_disk_layout}" && \
+      "${FLAGS_disk_layout}" != "default" ]]; then
+    DISK_LAYOUT_TYPE="${FLAGS_disk_layout}"
+  fi
 }
 
 emit_gpt_scripts() {