Allow disabling legacy properties

Introduce TW_NO_LEGACY_PROPS flag for users that build TWRP in
non-kitkat trees and don't need the legacy prop environment.

Change-Id: Iacb2d9af7d7069acb3d0a96bb34f32ded6317a20
diff --git a/Android.mk b/Android.mk
index 46098cc..7ac4d41 100644
--- a/Android.mk
+++ b/Android.mk
@@ -272,6 +272,9 @@
 else ifeq ($(TARGET_RECOVERY_QCOM_RTC_FIX),true)
     LOCAL_CFLAGS += -DQCOM_RTC_FIX
 endif
+ifneq ($(TW_NO_LEGACY_PROPS),)
+	LOCAL_CFLAGS += -DTW_NO_LEGACY_PROPS
+endif
 ifneq ($(wildcard bionic/libc/include/sys/capability.h),)
     LOCAL_CFLAGS += -DHAVE_CAPABILITIES
 endif