build: make coreboot building an else in "roms"

Otherwise, the current return prevents set -u -e
after the case/switch block, which is a problem if
set +u +e was done at any point before the return.

Remove the return in the roms) section of the case/switch
block, and make the building of coreboot images part of
an else clause.

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

build

@@ -47,10 +47,10 @@ main()
 	roms)
 		if [ $# -gt 1 ] && [ "$2" = "serprog" ]; then
 			mk -b stm32-vserprog pico-serprog
-			return 0
+		else
-		fi
+			shift 1
-		shift 1
+			x_ ./mk -b coreboot "$@"
-		x_ ./mk -b coreboot "$@" ;;
+		fi ;;
 	*)
 		[ -f "$spath" ] || $err "bad command"
 		$shcmd