[coreboot] buildrom: cleanup package includes (updated)

[Jordan Crouse]

Attached is an updated version of the patch I sent before that 
streamined the .mk files that are included in buildrom.  This version
is updated with the latest and greatest from Myles and everybody.

Tested with all payloads + coreinfo, and hd8mr + kernel (I'm going to
test the other platforms with kernel, but I didn't want to wait around
for them to finish building).

Jordan