Support target-specific sdk/build/tools.atree extension.

The purpose of this patch is to add the ability to specify a
target-specific extension to sdk/build/tools.atree, under the
name sdk/build/tools.$(TARGET_ARCH).atree

This is needed to move x86-specific changes out of tools.atree
in order to fix the build of internal Android branches that
don't include other x86-related changes to the build system.

Another patch, following this patch, will fix the build itself
by moving the x86-stuff to sdk/build/tools.x86.atree.

NOTE: The root cause of the problem is that tools.atree
probably shouldn't be in the public AOSP repository, but
under development/build/ instead. However, fixing this
requires more drastic changes to the build system.

Change-Id: Ie365c55527bcad38e1e5248f618e2fd5a96431a1

core/Makefile

@@ -1318,11 +1318,21 @@ endif
 
 atree_dir := development/build
 
+# sdk/build/tools.atree contains the generic rules, while
+#
+# sdk/build/tools.$(TARGET_ARCH).atree contains target-specific rules
+# the latter is optional.
+#
+sdk_tools_atree_files := sdk/build/tools.atree
+ifneq (,$(strip $(wildcard sdk/build/tools.$(TARGET_ARCH).atree)))
+  sdk_tools_atree_files += sdk/build/tools.$(TARGET_ARCH).atree
+endif
+
 sdk_atree_files := \
 	$(atree_dir)/sdk.exclude.atree \
 	$(atree_dir)/sdk.atree \
 	$(atree_dir)/sdk-$(HOST_OS)-$(HOST_ARCH).atree \
-	sdk/build/tools.atree
+	$(sdk_tools_atree_files)
 
 deps := \
 	$(target_notice_file_txt) \
@@ -1336,7 +1346,7 @@ deps := \
 	$(INSTALLED_BUILD_PROP_TARGET) \
 	$(ATREE_FILES) \
 	$(atree_dir)/sdk.atree \
-	sdk/build/tools.atree \
+	$(sdk_tools_atree_files) \
 	$(HOST_OUT_EXECUTABLES)/atree \
     $(HOST_OUT_EXECUTABLES)/line_endings