RAK4631 support

[maxgerhardt]

Adds support for the Wiscore RAK4631 Board and its (from Adafruit) forked core https://github.com/RAKWireless/RAK-nRF52-Arduino in the latest 1.3.3. version. Following from community discussion.

Attempts to supersede the weird "live-patch" method that RAK does with https://docs.rakwireless.com/Knowledge-Hub/Learn/Board-Support-Package-Installation-in-PlatformIO/#update by introducing a proper package + board definition into mainline.

The package version is the 1.3.3 version with backpatches for the PlatformIO-specific library scripts (https://github.com/maxgerhardt/RAK-nRF52-Arduino/commit/547e46cc8e0a5b907dd0728fbc32b718396b6c46, https://github.com/maxgerhardt/RAK-nRF52-Arduino/commit/758aff9a9b440a1664a76b00532443e05475dff4, https://github.com/maxgerhardt/RAK-nRF52-Arduino/commit/817eadbdf0e7083e1175feb1aba926b2cc75ee7f).

Blink example and Bluefruit example compile normally.

Suggested course of action:

1. Merge https://github.com/platformio/builder-framework-arduino-nrf5/pull/14
2. Readjust builder/frameworks/arduino submodule target here
3. Add framework package to registry, adjust platform.json
4. Merge this PR

To test this PR on its own:

1. Create any PlatformIO project (e.g., Board: "Arduino Uno", Framework "Arduino")
2. Overwrite generated platformio.ini with

[env:wiscore_rak4631]
platform = https://github.com/maxgerhardt/platform-nordicnrf52.git#rak
framework = arduino
board = wiscore_rak4631

3. Press build