cros_make_image_bootable, mount_gpt_image, common.sh: root filesystem changes: ext2, ro by default

mount_gpt_image.sh

Index: mount_gpt_image.sh
diff --git a/mount_gpt_image.sh b/mount_gpt_image.sh
index ee7b3cc6f60de363332c1434e045b212a4ffec35..be95dc5dcf3d0fda1e94cd27963f02b7f5ac967a 100755
--- a/mount_gpt_image.sh
+++ b/mount_gpt_image.sh
@@ -79,7 +79,14 @@ function get_gpt_partitions() {
   # Mount the rootfs partition using a loopback device.
   local offset=$(partoffset "${FLAGS_from}/${filename}" 3)
   local ro_flag=""
-  [ ${FLAGS_read_only} -eq ${FLAGS_TRUE} ] && ro_flag="-o ro"
+  if [ ${FLAGS_read_only} -eq ${FLAGS_TRUE} ]; then
+    ro_flag="-o ro"
+  else
+    # Make sure any callers can actually mount and modify the fs
+    # if desired.
+    # cros_make_image_bootable should restore the bit if needed.
+    enable_rw_mount "${FLAGS_from}/${filename}" "$(( offset * 512 ))"
+  fi
 
   sudo mount ${ro_flag} -o loop,offset=$(( offset * 512 )) \
     "${FLAGS_from}/${filename}" "${FLAGS_rootfs_mountpt}"