vendor.sh: Allow setmac if vendorfiles not needed

Observe the following prior patch:

commit 818f3d630c
Author: Leah Rowe <leah@libreboot.org>
Date:   Fri Jan 3 17:06:14 2025 +0000

    vendor.sh: Don't error if vcfg is unset

Now:

This patch made vendor inject more robust, and speeds
up the processing of images where no vendor files are
needed, but it broke setmac on such tar archives.

This new patch works around it. For example, I was
able to run ./mk inject on an X200 tarball to change
the MAC address; no vendorfiles are inserted, because
it's not needed.

The further check for whether a board uses Intel GbE
still protects against accidental modification.

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

include/vendor.sh

@@ -358,7 +358,7 @@ vendor_inject()
 
 	check_release "$archive" || \
 	    $err "You must run this script on a release archive. - $dontflash"
-	if readcfg; then
+	if readcfg || [ -n "$new_mac" ]; then
 		[ "$nukemode" = "nuke" ] || x_ ./mk download "$board"
 		patch_release_roms
 	else