cbfs: look for CBFS header in a predefined place

This patch introduces a new option (CONFIG_MULTIPLE_CBFS_INSTANCES) to
allow multiple CBFS instances in the bootrom.

When the new option is enabled, the code running on the target
controls which CBFS instance is used. Since all other then header CBFS
structures use relative addressing, the only value which needs
explicit setting is the offset of the CBFS header in the bootrom.

This patch adds a facility to set the CBFS header offset. The offset
value of zero means default. i.e. the CBFS initialization code still
discovers the offset through the value saved at the top of the ROM.

BRANCH=storm
BUG=chrome-os-partner:34161, chromium:445938
TEST=with the rest patches in, storm target successfully boots from RW
     section A.

Change-Id: Id8333c9373e61597f0c653c727dcee4ef6a58cd2
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: e57a3a15bba7cdcca4a5d684ed78f8ac6dbbc95e
Original-Change-Id: I4c026389ec4fbaa19bd11b2160202282d2f9283c
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/237569
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/9747
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Edward O'Callaghan <edward.ocallaghan@koparo.com>

src/include/memlayout.h

@@ -102,4 +102,6 @@
 		. += sz;
 #endif
 
+#define CBFS_HEADER_OFFSET(addr) REGION(cbfs_header_offset, addr, 4, 4)
+
 #endif /* __MEMLAYOUT_H */