safe-upgrade: fix calibration data partition name

DISCLAIMER: Breaks safe-upgrade retrocompatibility!

Newer OpenWrt rely on calibration data MTD partitions being named 'art'
older OpenWrt on which librerouterOS 1.5 is based, looked for calibration
calibration data for on-SoC radio on 'ART' partition.
In particular librerouter v1 have an on SoC radio which have calibration data
in art partition, safe-upgrade can either support older OpenWrt version or
newer, but not both at least without change the code drastically.
Here in development branch let's support new OpenWrt.

packages/safe-upgrade/files/usr/sbin/safe-upgrade

@@ -346,7 +346,7 @@ local function upgrade(args)
     else
         fw_mtd_str = '7936k(firmware),7936k(fw2)'
     end
-    local boot_script_tpl = 'set bootargs console=ttyS0,115200 board=LIBREROUTERV1 mtdparts=spi0.0:256k(u-boot),64k(u-boot-env),%s,128k(res),64k(ART); echo booting part %d; bootm ${fw_addr};'
+    local boot_script_tpl = 'set bootargs console=ttyS0,115200 board=LIBREROUTERV1 mtdparts=spi0.0:256k(u-boot),64k(u-boot-env),%s,128k(res),64k(art); echo booting part %d; bootm ${fw_addr};'
     local boot_script = string.format(boot_script_tpl, fw_mtd_str, partitions.other)
     set_uboot_env(string.format('boot_%d', partitions.other), boot_script)
     set_testing_partition(partitions.other)