base-files: safer sysupgrade.tar for kernel-out-of-UBI

Ensure that the kernel CRC is invalidated while rootfs is being updated.
This allows the bootloader to detect an interrupted sysupgrade and fall
back to an alternate booting method, such as TFTP, instead of just going
ahead with normal boot and effectively bricking the device.

Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>

package/base-files/files/lib/upgrade/nand.sh

@@ -303,7 +303,7 @@ nand_upgrade_tar() {
 	local has_env=0
 
 	[ "$kernel_length" != 0 -a -n "$kernel_mtd" ] && {
-		tar xf "$tar_file" ${board_dir}/kernel -O | mtd write - $CI_KERNPART
+		mtd erase $CI_KERNPART
 	}
 	[ "$kernel_length" = 0 -o ! -z "$kernel_mtd" ] && has_kernel=
 	[ "$CI_KERNPART" = "none" ] && has_kernel=
@@ -317,6 +317,10 @@ nand_upgrade_tar() {
 			ubiupdatevol /dev/$root_ubivol -s $rootfs_length -
 	}
 
+	[ "$kernel_length" != 0 -a -n "$kernel_mtd" ] && {
+		tar xf "$tar_file" ${board_dir}/kernel -O | \
+			mtd -n write - $CI_KERNPART
+	}
 	[ "$has_kernel" = "1" ] && {
 		local kern_ubivol="$( nand_find_volume $ubidev $CI_KERNPART )"
 		tar xf "$tar_file" ${board_dir}/kernel -O | \