trees: call err if multiple flags are provided

this script is designed to only run a single flag.

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

script/trees

@@ -17,6 +17,7 @@ eval `setvars "" xarch cdir config config_name xlang mode makeargs \
 main()
 {
 	while getopts f:b:m:u:c:x:s:l:n: option; do
+		[ -n "$_f" ] && $err "only one flag is permitted"
 		_f="$1"
 		case "$1" in
 		-b) mode="" ;;