[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>