apm821xx: use x86's upgrade scripts for MyBook Live

apm821xx: use x86's upgrade scripts for MyBook Live

Advantages:
 - preserves existing partition layout. On the hard-drive.
   Only the boot and rootfs partition will be overwritten.

Disadvantages:
 - The upgrade process takes much longer to run.
   from 2-3 seconds to 15-25 seconds.

Please note that sysupgrade will refuse to upgrade, if the existing
installation has an incompatible partition layout. Future changes
to the bootfs and/or rootfs partition size will likely cause breakage
to the sysupgrade procedure. In these cases, the ext4-rootfs.img.gz
has to be written manually onto the disk. Please don't forget to backup
your configuration in this cases.

Note2: This patch requires
"base-files: upgrade: make get_partitions() endian agnostic"

Note3: If your current installation does not host the two
changes, sysupgrading will wipe the existing partition
layout. Don't forget to backup your data!

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>