base-files: safer sysupgrade for kernel-in-UBI devices

base-files: safer sysupgrade for kernel-in-UBI devices

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, instead of just going ahead with
normal boot and effectively bricking the device.

Possible fallbacks include a recovery initramfs partition or UBI volume
and TFTP. See here for an example U-Boot configuration with fallbacks:
https://shorturl.at/befsA (https://github.com/Lanchon/openwrt-tr4400-v2/
blob/e7d707d6bd7839fbd0b8d0bd180fce451df77e47/install-recovery.sh#L52-L63)

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