Document the various top-level Makefiles

Makefile.embedded

@@ -1,3 +1,8 @@
+# This Makefile contains make variables and rules that are only applicable
+# to builds for embedded devices (i.e. excluding platforms native and cooja).
+# Future extensions to the build system that are of a similar nature (for
+# embedded devices only), can be achieved by extending this Makefile here.
+
 RLWRAPGOALS = login serialdump serialview
 
 .PHONY: $(RLWRAPGOALS)

Makefile.identify-target

@@ -1,3 +1,8 @@
+# This Makefile can be used to identify the selected TARGET used for a
+# specific build. It can be included by example Makefiles that need to take
+# decisions based on TARGET. It is also automatically included by the
+# top-level Makefile.include.
+
 ifeq ($(TARGET),)
   -include Makefile.target
   ifeq ($(TARGET),)

Makefile.tools

@@ -1,3 +1,7 @@
+# Some make rules in the main build system depend on the presence of utilities
+# under the tools/ dir. For those dependencies, we use this makefile here to
+# recursively invoke the respective build under tools/.
+
 TOOLS_DIR = $(CONTIKI)/tools
 TOOL_DEPS = $(TOOLS_DIR)/tools-utils.c $(TOOLS_DIR)/tools-utils.h