Hardcoded RXD/TXD pin

[mla-martin]

For the serial bootloader mode the RXD and TXD pins are hardcoded in sdk_config.h and changes in the boards/ folder do not propagate to the nordic SDK part of the bootloader.
I propose using the RXD/TXD/RTS/CTS pin definitions from the board headers in the sdk_config.h

[hathach]

thanks, we just modify the code to use RXD defined in boad header. Only 832 will use that though, 840 will use usb cdc.