[BUILD] Generate include/autoconf.mk from board config files

Use cpp and sed to postprocess config.h and import the defined values
into include/autoconf.mk.  autoconf.mk is then included by config.mk to
give 'make' access to the board configuration.

Doing this enables conditional compilation at the Makefile level instead
of by wrapping every .c file with #ifdef/#endif wrappers.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/config.mk b/config.mk
index 37d61a0..79e5a31 100644
--- a/config.mk
+++ b/config.mk
@@ -87,6 +87,9 @@
 sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk	# include board specific rules
 endif
 
+# Load generated board configuration
+sinclude $(OBJTREE)/include/autoconf.mk
+
 #########################################################################
 
 CONFIG_SHELL	:= $(shell if [ -x "$$BASH" ]; then echo $$BASH; \