make help: prints a handy list of useful targets

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

Change-Id: I8f6cebe6cbd4cf89736da23e9d0d2a257ea422ee
diff --git a/core/main.mk b/core/main.mk
index 1c7706a..91e5cc4 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -54,6 +54,9 @@
 .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 @@
 .PHONY: showcommands
 showcommands:
 	@echo >/dev/null
+