luarpc hangs and never connects

[JohnTordur]

When running ./luarpc after making it build using the fix mention in #173
It hangs consistently after specifying the correct serial port.
I have tested this both on mac-os and ubuntu 20 lts.
I'm testing on stm32f411re nucleo64 board.
I'm using gcc-arm-none-eabi compiler for my cross builds.

handle, err = rpc.connect("/dev/ttyACM0") hangs
Building the ordinary firmware works, and it boots straight to the CLI over the serial port.

[bogdanm]

Please retry with the latest master, but also see comment in #173.