make help: prints a handy list of useful targets

A small step to making the platform build environment more
self-documenting.

Change-Id: I8f6cebe6cbd4cf89736da23e9d0d2a257ea422ee

core/help.mk

@@ -0,0 +1,42 @@
+#
+# Copyright (C) 2010 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+ifeq ($(MAKECMDGOALS),help)
+dont_bother := true
+endif
+ifeq ($(MAKECMDGOALS),out)
+dont_bother := true
+endif
+
+.PHONY: help
+help:
+	@echo
+	@echo "Common make targets:"
+	@echo "----------------------------------------------------------------------------------"
+	@echo "droid                   Default target"
+	@echo "clean                   (aka clobber) equivalent to rm -rf out/"
+	@echo "snod                    Quickly rebuild the system image from built packages"
+	@echo "offline-sdk-docs        Generate the HTML for the developer SDK docs"
+	@echo "doc-comment-check-docs  Check HTML doc links & validity, without generating HTML"
+	@echo "libandroid_runtime      All the JNI framework stuff"
+	@echo "framework               All the java framework stuff"
+	@echo "services                The system server (Java) and friends"
+	@echo "help                    You're reading it right now"
+
+.PHONY: out
+out:
+	@echo "I'm sure you're nice and all, but no thanks."
+

core/main.mk

@@ -54,6 +54,9 @@ $(DEFAULT_GOAL):
 .PHONY: FORCE
 FORCE:
 
+# Targets that provide quick help on the build system.
+include $(BUILD_SYSTEM)/help.mk
+
 # Set up various standard variables based on configuration
 # and host information.
 include $(BUILD_SYSTEM)/config.mk
@@ -842,3 +845,4 @@ modules:
 .PHONY: showcommands
 showcommands:
 	@echo >/dev/null
+