Add sigrok-firmware-fx2lafw to MXE build dependencies and fix sigrok-firmware build
Add dockerfile for the MXE build
This is version 0.2 of the MXE dockerfile. It is based on MXE commit
b48b3cc7 and contains both the i686-w64-mingw32.static.posix and
x86_64-w64-mingw32.static.posix target.