realtek: Migrate to libdeflate

realtek: Migrate to libdeflate

Libdeflate is a more advanced gzip compressor, which allows for faster
decompression, higher compression speed (factor 3-4), while being fully
gzip compatible.

Some comparison
gzip    | libdeflate-gzip | delta  | image [openwrt-realtek-rtl839x-*]
--------+-----------------+--------+-----------------------------------------------
6589174 | 6298794         | 290380 | d-link_dgs-1210-52-initramfs-kernel.bin
6291632 | 6029488         | 262144 | d-link_dgs-1210-52-squashfs-factory_image1.bin
6292270 | 6030128         | 262142 | d-link_dgs-1210-52-squashfs-sysupgrade.bin
6589142 | 6298760         | 290382 | zyxel_gs1900-48-initramfs-kernel.bin
6292264 | 6030122         | 262142 | zyxel_gs1900-48-squashfs-sysupgrade.bin

and changing lzma to (libdeflate-)gzip on existing rtl930x target:
gzip    | libdeflate-gzip | delta  | image [openwrt-realtek-rtl930x-*]
--------+-----------------+--------+--------------------------------------
6816230 | 6510382         | 305848 | zyxel_xgs1250-12-initramfs-kernel.bin

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Reviewed-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Sander Vanheule <sander@svanheule.net>