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

Android.mk

@@ -166,6 +166,10 @@ ifeq ($(PRODUCT_USE_DYNAMIC_PARTITIONS),true)
     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