build/fw/coreboot: reset grub background each time

the script used to be called once per target, now it
handles every target. the grub background image wasn't
being set, so if it changed at build time, it would
stay changed.

keep the default in place for each run, while still
allowing target.cfg files to change it per target.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/build/fw/coreboot

@@ -51,6 +51,7 @@ main()
 	for x in ${boards}; do
 		eval "$(setvars "n" ${pv})"
 		eval "$(setvars "" ${v})"
+		grub_background="background1280x800.png"
 		board="${x}"
 		check_target
 		prepare_target