releasetools: Fix the bug with TARGET_NO_RECOVERY.

The packaging script is broken when we set TARGET_NO_RECOVERY and
SYSTEM_ROOT_IMAGE both. With TARGET_NO_RECOVERY set, we don't have
RECOVERY/RAMDISK/etc/recovery.fstab. It thus fails the assertion that
"/" must exist in fstab when enabling SYSTEM_ROOT_IMAGE.

Change-Id: Id509d724f18be7cf7389dd786924ead894ac0cc1
(cherry picked from commit 48550cce)