In the BareboxDriver, UBootDriver, and SmallUBootDriver, the option sent to the bootloader to interrupt boot loading is done with some form of:

self.console.write(interrupstring.encode('ASCII'))

According to the documentation

use \x03 for CTRL-C
or
use “\x03” for CTRL-C

Neither of these work.

After using labgrid-client -vv I see the following is written to the console

CONSOLE SerialLogger.ma:   SerialDriver(main) > \x03␍␤
DEBUG   SerialDriver(ma:   Write 4 bytes: b'\\x03'

or

CONSOLE SerialLogger.ma:   SerialDriver(main) > \\x03␍␤
DEBUG   SerialDriver(ma:   Write 5 bytes: b'\\\\x03'

Neither of which is correct and causing the boot loader not to be interrupted.