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
diff --git a/gui/Android.mk b/gui/Android.mk
index 3e499ab..113fa53 100644
--- a/gui/Android.mk
+++ b/gui/Android.mk
@@ -74,9 +74,9 @@
 $(error stopping)
 endif
 
-ifeq "$(wildcard bootable/recovery/gui/devices/$(DEVICE_RESOLUTION))" ""
+ifeq "$(wildcard $(commands_recovery_local_path)/gui/devices/$(DEVICE_RESOLUTION))" ""
 $(warning ********************************************************************************)
-$(warning * DEVICE_RESOLUTION ($(DEVICE_RESOLUTION)) does NOT EXIST in bootable/recovery/gui/devices )
+$(warning * DEVICE_RESOLUTION ($(DEVICE_RESOLUTION)) does NOT EXIST in $(commands_recovery_local_path)/gui/devices )
 $(warning * Please choose an existing theme or create a new one for your device )
 $(warning ********************************************************************************)
 $(error stopping)