board/BuR/brppt1: convert brppt1 boards to driver model

- add a devicetree for each variant (mmc, spi, nand)
- drop unneeded code from board and bur/common
- drop unneeded stuff from config header files
- minor adaptions to be compliant with driver model (requesting gpio,..)
- harmonize the commandset over all brppt1 targets

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>