Reliably build arbitrary Kconfig-based revisions of SeaBIOS

Reliably build arbitrary Kconfig-based revisions of SeaBIOS

Reliability is accomplished by checking out the desired SeaBIOS commitish
into a branch named 'coreboot' in the local SeaBIOS git repository. Using
a branch allows TAG-$(CONFIG_SEABIOS_..) to refer to any commitish in the
SeaBIOS git repo, not just branches and tags.

Configuration is done with make defconfig followed by enabling and
disabling of the relevant coreboot-specific SeaBIOS options by appending
to .config using echo. This works, because later entries in .config will
overwrite earlier ones.

Signed-off-by: Peter Stuge <peter@stuge.se>
Acked-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6335 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1