efi_loader: Fall back to fdtfile naming convention

efi_loader: Fall back to fdtfile naming convention

When there is no $fdtfile variable set, we still have a good chance
that on 32bit arm the fdtfile really is just called $soc-$board.dtb.

Enable the exports for $soc and $board in our distr defaults and make
use of them in the efi boot script.

Reported-by: Andreas Faerber <afaerber@suse.de>
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>