mvebu: sysupgrade: drop unnecessary UBI to UBI logic

mvebu: sysupgrade: drop unnecessary UBI to UBI logic

The recent changes to the maximum kernel size for Mamba and Venom
highlighted the fact that the old Mamba kernel size has been
hardcoded in linksys_get_root_magic() even for devices with
a different kernel/rootfs split.

The purpose of this code seems to be to avoid issues caused
by partially overwriting an existing UBI partition, where some
of the erase counters would be reset but not the unmodified
ones.  This problem has been solved in a more generic way by
the UBI EOF marker. This ensures that any old PEBs after the
marker are properly initialized.  It is therefore unnecessary
to erase the whole partition before flashing a new OpenWrt
factory image.

Signed-off-by: Bjørn Mork <bjorn@mork.no>