ath79: lzma-loader: sync with ar71xx target

ath79: lzma-loader: sync with ar71xx target

Import all improvements made in the lzma-loader since development on the
ath79 target started.

This also reverts fe594bf90d09 ("ath79: fix loader-okli, lzma-loader"), as
is obsoleted by 2ad60168b6af ("ar71xx: lzma-loader: move padding workaround
to gzip step").

Likely, many of the changes should be ported to the ramips lzma-loader as
well, but I don't have a device to test this.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>