Added the test for nRF52840-DK board with MCUBoot variant.
Images based on MCUBoot may be used for test automation since this
bootloader type does not have version dependencies.
To prepare the board for testing it should be flashed with MCUBoot
image variant with `nrfjprog` initially.
This allows us to do a few things:
* Remove the runtime dep on Python 3, which is tricky for ChromeOS
* Test composite devices more efficiently, only writing once per test
* Automatically upload signed reports for successful device tests.
