System_Property_Get: Allow specifying multiple prop file paths

Some OEMs include prop files in custom locations. Add these to
a list and parse when setting properties.

Use the flag TW_SYSTEM_BUILD_PROP_ADDITIONAL_PATHS followed
by a semi-colon-separated list of paths (relative to /system)
where the additional prop files can be found.

The standard build.prop file does not need to be specified and
will always be parsed.

Example:
TW_SYSTEM_BUILD_PROP_ADDITIONAL_PATHS := etc/buildinfo/oem_build.prop;etc/prop.default

Change-Id: Ie0e25c7d2575d928310ff1b4fc1aef44a83784ca
(cherry picked from commit a185252c8e6d624587bc3d17f005cc122bda6cf7)
diff --git a/Android.mk b/Android.mk
index 42c4053..c2bca79 100755
--- a/Android.mk
+++ b/Android.mk
@@ -163,6 +163,10 @@
     endif
 endif
 
+ifneq ($(TW_SYSTEM_BUILD_PROP_ADDITIONAL_PATHS),)
+    LOCAL_CFLAGS += -DTW_SYSTEM_BUILD_PROP_ADDITIONAL_PATHS='"$(TW_SYSTEM_BUILD_PROP_ADDITIONAL_PATHS)"'
+endif
+
 ifeq ($(TW_USES_VENDOR_LIBS),true)
     LOCAL_CFLAGS += -DUSE_VENDOR_LIBS=1
 endif