new variable TARGET_RECOVERY_FSTAB

Currently the source file recovery.fstab has to be located
in $(TARGET_DEVICE_DIR)/recovery.fstab).

To allow multiple similar CM products to share one recovery.fstab
file this patch introduces the variable TARGET_RECOVERY_FSTAB.

Change-Id: I51961f5a1665023001c9857e13008a2ca44529ee