Update to 5.2.3 and sync URLs with LEDE

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>

Update xz to version 5.2.2.
Add a secondary download location.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

Signed-off-by: Alexander Ryzhov <openwrt@ryzhov-al.ru>

- update to latest version (v5.2.1)
 - add license info
 - add myself as maintainer
 - put everything in a "Compression" submenu
 - reduce the number of packages (put symlinks with their matching target)

Signed-off-by: Nicolas Thill <nico@openwrt.org>