Make bootloader warning message configurable

[microbit-mark]

Description: https://github.com/ARMmbed/DAPLink/pull/401 introduced an error message to identify to users that their machine is booting from USB DAPLink. For micro:bit board users with less technical knowledge, the message is intimidating and hard to figure out that it relates to the micro:bit.

Feature: Make the message configurable per board as per suggestion https://github.com/ARMmbed/DAPLink/pull/401#issuecomment-464367948

Alternatives: If it can't be per device, make the message more explicit eg 'Cannot boot from ARM DAPLINK device. Please remove any DAPLINK devices (e.g. micro:bit) from USB sockets and restart this computer'