orion: sysupgrade: change platform_check_image() args validation

sysupgrade passes image to check as argument so use $# instead of $ARGC.
It also fits this function better as it checks $1 and not $ARGV.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/orion/generic/base-files/lib/upgrade/platform.sh

@@ -6,7 +6,7 @@
 # use default for platform_do_upgrade()
 
 platform_check_image() {
-	[ "${ARGC}" -gt 1 ] && { echo 'Too many arguments. Only flash file expected.'; return 1; }
+	[ "$#" -gt 1 ] && { echo 'Too many arguments. Only flash file expected.'; return 1; }
 
 	local hardware="$(board_name)"
 	local magic="$(get_magic_word "$1")"