build/boot/roms: only do 1 custom kbd/payload/mode

-k, -p and -d let you set keymap, payload and displaymode
respectively, but the handling for this is buggy when
passing multiple arguments.

Support only one argument, for simplicity. This is how
people use them anyway, and it makes lbmk less buggy.

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

include/boot.sh

@@ -3,11 +3,12 @@
 # SPDX-FileCopyrightText: 2022 Ferass El Hafidi <vitali64pmemail@protonmail.com>
 # SPDX-FileCopyrightText: 2023 Leah Rowe <leah@libreboot.org>
 
+first=""
 board=""
 boards=""
-displaymodes=""
-payloads=""
-keyboard_layouts=""
+_displaymode=""
+_payload=""
+_keyboard_layout=""
 
 main()
 {
@@ -21,13 +22,13 @@ main()
 	while [ $# -gt 0 ]; do
 		case ${1} in
 		-d)
-			displaymodes="${2} ${displaymodes}"
+			_displaymode="${2}"
 			shift ;;
 		-p)
-			payloads="${2} ${payloads}"
+			_payload="${2}"
 			shift ;;
 		-k)
-			keyboard_layouts="${2} ${keyboard_layouts}"
+			_keyboard_layout="${2}"
 			shift ;;
 		all)
 			first="all" ;;