main.mk: Allow overriding subdirs via make variable TARGET_SUBDIRS

Change-Id: I9e1523833e1567a59b19482c7834fe30479a009f
Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/core/main.mk b/core/main.mk
index 569d4dc..9b05f76 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -480,11 +480,16 @@
 	external/zlib
 else	# !BUILD_TINY_ANDROID
 
+ifneq ($(TARGET_SUBDIRS),)
+subdirs := $(TARGET_SUBDIRS)
+else
 #
 # Typical build; include any Android.mk files we can find.
 #
 subdirs := $(TOP)
 
+endif	# !TARGET_SUBDIRS
+
 FULL_BUILD := true
 
 endif	# !BUILD_TINY_ANDROID