asusuimage: new tool for creating TRX-images compatible with AsusWRT

asusuimage: new tool for creating TRX-images compatible with AsusWRT

This tool allows you to create `trx`-images for many Asus routers.
Supported v2 and v3 format of `trx`-images.

This format and tool use the mkimage generated uImage or multi images
and add Asus specific tail to pass AsusWRT validation.

Tested on Asus RT-AX89X which uses the v2 format.

Signed-off-by: Oleg S <remittor@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
[Major code cleanup, use generic endian conversion instead of defines,
use switches, move info printing to a separate function, use zlib CRC32]
[Remove VSC compatibility, fix warnings, rename to asusimage,
move variables to top of functions]
Link: https://github.com/openwrt/firmware-utils/pull/31
Signed-off-by: Robert Marko <robimarko@gmail.com>