libpayload: Expand doxygen definitions

libpayload:  Expand doxygen definitions

Expand libpayload.h to include a main page and add individual
groups for the API functions - this adds the Modules tab to
the doxygen output.

Specify the INPUT list rather then the EXCLUDE list of directories
to omit random .c files that we don't want.  Add clean targets to
the makefile to clean doxygen files only.

Signed-off-by: Jordan Crouse <jordan.crouse@amd.com>
Acked-by: Stefan Reinauer <stepan@coresystems.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3545 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1