xiaomifw: new tool for managing Xiaomi firmwares

xiaomifw: new tool for managing Xiaomi firmwares

This has been tested with Xiaomi routers images (r1cl, r1cm, r2100, r3,
r3d, r3g, r4cm, ra70) but it should also be compatible with other
devices as Xiaomi seems to share the same format across most of their
products.

The biggest & probably unresolvable problem is RSA 256 B signature.
Without the key it's impossible to create an image that official UI
would accept.

This tool may be still useful for flashing original Xiaomi firmwares
(e.g. when reverting to official image) and for development purposes.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>