Make bootloader warning message configurable

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'

Start by reading PR #401 and its linked comment, then locate the bootloader warning message in the DAPLink firmware sources. Done means the warning can be configured per board, or the agreed explicit alternative message is implemented; no file or test path is named in the issue.

Written by the indexing model from the issue text.