coreboot: introduce romstage_handoff structure

coreboot: introduce romstage_handoff structure

The romstage_handoff structure is intended to be a way for romstage and
ramstage to communicate with one another instead of using sideband
signals such as stuffing magic values in pci config or memory
scratch space. Initially this structure just contains a single region
that indicates to ramstage that it should reserve a memory region used
by the romstage. Ramstage looks for a romstage_handoff structure in cbmem
with an id of CBMEM_ID_ROMSTAGE_INFO. If found, it will honor reserving
the region defined in the romstage_handoff structure.

Change-Id: I9274ea5124e9bd6584f6977d8280b7e9292251f0
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2791
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>