allow using coreboot's build system to add payload

lbmk must still define payloads, but specific configs
may use coreboot's build system instead.

you might use this to add your own config with, say,
tianocore payload, using coreboot.git to build it,
rather than using lbmk's choice of payloads.

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

include/vendor.sh

@@ -248,7 +248,8 @@ detect_board()
 	path="$1"
 	filename="$(basename "$path")"
 	case "$filename" in
-	grub_*|seagrub_*) board="$(echo "$filename" | cut -d '_' -f2-3)" ;;
+	grub_*|seagrub_*|custom_*)
+		board="$(echo "$filename" | cut -d '_' -f2-3)" ;;
 	seabios_withgrub_*)
 		board="$(echo "$filename" | cut -d '_' -f3-4)" ;;
 	*.tar.xz)