util/abuild: Only update submodules once

Without this, each build will try to update the submodules. Not
necessarily a problem but git locks repos, creating spurious error
messages.

Change-Id: Iba20677d4b5f9365c92f7ed247ca56acb7d33b27
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

util/abuild/abuild

@@ -796,8 +796,11 @@ if [ "$cpus" != "1" ]; then
 	echo | xargs -P ${cpus:-0} -n 1 echo 2>/dev/null >/dev/null && USE_XARGS=1
 fi
 
+git submodule update --checkout --init
+
 if [ "$USE_XARGS" = "0" ]; then
 test "$MAKEFLAGS" == "" && test "$cpus" != "" && export MAKEFLAGS="-j $cpus"
+export MAKEFLAGS="$MAKEFLAGS UPDATED_SUBMODULES=1" # no need to re-download
 build_targets()
 {
 	local targets=${*-$(get_mainboards)}