base-files: emmc: invalidate kernel image while rootfs is being written

base-files: emmc: invalidate kernel image while rootfs is being written

Some setups have alternate boot actions in case the main OS fails to
boot. These can include a secondary copy of the OS, a recovery OS, a
fallback to TFTP boot, etc.

This commit invalidates the kernel image while rootfs is being written
which, if a sysupgrade is interrupted, will trigger an alternate boot
action in devices that support it. This results in safer sysupgrades.

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