treewide: sysupgrade: don't use $ARGV in platform_do_upgrade()

stage2 passes image path to platform_do_upgrade() as an argument so it
can be simply accessed using $1

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 8b4bc7abe0)
[rmilecki: dropping ARGV without this change broke sysupgrade]
Fixes: 6ac62c4b6c ("base-files: don't set ARGV and ARGC")

target/linux/ramips/base-files/lib/upgrade/platform.sh

@@ -27,7 +27,7 @@ platform_nand_pre_upgrade() {
 	case "$board" in
 	ubnt-erx|\
 	ubnt-erx-sfp)
-		platform_upgrade_ubnt_erx "$ARGV"
+		platform_upgrade_ubnt_erx "$1"
 		;;
 	esac
 }
@@ -43,14 +43,14 @@ platform_do_upgrade() {
 	ubnt-erx-sfp|\
 	xiaomi,mir3g|\
 	xiaomi,mir3p)
-		nand_do_upgrade "$ARGV"
+		nand_do_upgrade "$1"
 		;;
 	tplink,c50-v4)
 		MTD_ARGS="-t romfile"
-		default_do_upgrade "$ARGV"
+		default_do_upgrade "$1"
 		;;
 	*)
-		default_do_upgrade "$ARGV"
+		default_do_upgrade "$1"
 		;;
 	esac
 }