This is supposed to fix a problem where you can end up with two
non-working images.

This can be triggered by the following actions:

  * Starting with rootfs A and fitImage_active
  * Doing an update via swupdate using a corrupted kernel image
  * fitImage_active gets renamed to fitImage_inactive.
  * The corrupted kernel now in fitImage_active fails to boot
  * A fallback to rootfs A occurs: rootfs A is started together
    with the working fitImage_inactive
  * On the next update via swupdate the corrupted fitImage_active is
    renamed to fitImage_inactive ending up with two corrupted kernel
    images
  * The now corrupted fitImage_inactive is copied to fitImage_active
    and the boot is bricked

To remedy this flaw, simply swap the fitImage in sync with the rootfs.
Instead of the active/inactive image we now use A/B for the kernel image
and the currently used image is selected by the sys_active file.