sr: Allow device-specific recovery modules

Provide a means to include device-specific recovery modules that need to
be built (i.e. not pre-built). A list of recovery modules can be
specified by a device with:

TARGET_RECOVERY_DEVICE_MODULES := rec_mod_one rec_mod_two

Change-Id: Ibd19a71318863461d472471a4f7cfddfb1b9ae0d