Use --fast by default in build_image when --jobs is enabled.

build_image

Index: build_image
diff --git a/build_image b/build_image
index 9b94d99c5647889d739952f3e650b1a238e417a2..7b9abc7f187c1b1aa132fdc85f9d8b56705ea975 100755
--- a/build_image
+++ b/build_image
@@ -92,6 +92,11 @@ if [ "${FLAGS_rootfs_size}" -gt "${FLAGS_rootfs_partition_size}" ] ; then
   exit 1
 fi
 
+if [ ${FLAGS_jobs} -ne -1 ]; then
+  EMERGE_JOBS="--jobs=${FLAGS_jobs}"
+  FLAGS_fast=${FLAGS_TRUE}
+fi
+
 EMERGE_BOARD_CMD="emerge-${FLAGS_board}"
 if [ "${FLAGS_fast}" -eq "${FLAGS_TRUE}" ]; then
   echo "Using alternate emerge"
@@ -161,10 +166,6 @@ if [[ ${FLAGS_factory_install} -eq ${FLAGS_TRUE} ]] ; then
   INSTALL_MASK="${INSTALL_MASK} ${FACTORY_INSTALL_MASK}"
 fi
 
-if [[ ${FLAGS_jobs} -ne -1 ]]; then
-  EMERGE_JOBS="--jobs=${FLAGS_jobs}"
-fi
-
 # Figure out ARCH from the given toolchain.
 # TODO: Move to common.sh as a function after scripts are switched over.
 TC_ARCH=$(echo "${CHOST}" | awk -F'-' '{ print $1 }')