Build block TWRP with RECOVERY_VARIANT

Enable TWRP to reside alongside other recoveries with the naming
convention: bootable/recovery(-flag). If TWRP resides at
bootable/recovery and a device does not specify RECOVERY_VARIANT, then
it will build like normal. If TWRP resides at bootable/recovery-twrp,
then its makefiles will only be parsed if a device specifies
'RECOVERY_VARIANT := twrp'. This prevents TWRP specific makefile
warnings/errors (notably, missing DEVICE_RESOLUTION) when another
recovery is being built.

Change-Id: I8f02fffcd79c309c7123b9428eedc69af02e126e