brcm2708: use x86's upgrade scripts for all rpi targets

brcm2708: use x86's upgrade scripts for all rpi targets

Advantages:
 - preserves existing partition layout on the sd-card.
   Only the boot and rootfs partition will be overwritten.

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-sdcard.img.gz
will have to be written to the sdcard manually.
Please don't forget to backup your configuration in this case.

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