Fix entry overrides in support matrix generation
Fix entry overrides in support matrix generation

Fix #2539

The entries in the board's mpconfigboard.mk override any other entries.

Also, fixed a warning for bad use of backslash in a string.

adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython