According to the schematics user button should be on `PD5` not `PC5`
(so `GPIO_PIN(3, 5)`).
Due to the super high values resistors being used for the external
pullup it seems if you use a internal pullup it doesn't work.
The Sub-GHz Antenna of the Openmote-B is always connected to the AT86RF215.
It is the 2.4 GHz radial that can be switched between the CC2538 and the AT86RF215.
This changes the name of the defines accordingly.
It seems the doc was not added during the port.
This adds documentation similar to the openmote-cc2538.
This also expands on some flashing issues that have occurred.
