payloads/external/Makefile.inc: Don't rebuild SeaBIOS every time

Currently, if SeaBIOS is set as the payload, it gets rebuilt every
single time we do a build.

Change it to re-build just when there’s a config change.

Change-Id: Ib141f2cbf8796d449172432bb30fa4806cf90328
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13948
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed