sunxi: binman: Add U-Boot binary size check

The U-Boot binary may trip over its actual allocated size in the storage.
In such a case, the environment will not be readable anymore (because
corrupted when the new image was flashed), and any attempt at using saveenv
to reconstruct the environment will result in a corrupted U-Boot binary.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>