Cleanup access to vendor/part # info

Cleanup access to vendor/part # info

Instead of macros to access MAINBOARD record, use convenience functions.

Store pointers to MAINBOARD and HEADER for use outside of CB code.

Change-Id: I074e3a0df7d25726cbd942538bfdc5a63dd17e12
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Reviewed-on: http://review.coreboot.org/502
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>