Run 'git fetch' in SeaBIOS only when really needed

Run 'git fetch' in SeaBIOS only when really needed

This allows coreboot to compile without Internet connectivity

Change-Id: I969471e44e417f127fdc8744e868211500acee3e
Signed-off-by: Cristian Măgherușan-Stanciu <cristi.magherusan@gmail.com>
Reviewed-on: http://review.coreboot.org/11
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>