Add Boundary Devices Nitrogen6X boards

MAINTAINERS

board/boundary/nitrogen6x/1066mhz_4x128mx16.cfg

board/boundary/nitrogen6x/1066mhz_4x256mx16.cfg

board/boundary/nitrogen6x/6x_bootscript.txt

board/boundary/nitrogen6x/6x_bootscript_android.txt

board/boundary/nitrogen6x/6x_bootscript_android_recovery.txt

board/boundary/nitrogen6x/6x_upgrade.txt

board/boundary/nitrogen6x/800mhz_2x128mx16.cfg

board/boundary/nitrogen6x/800mhz_2x256mx16.cfg

board/boundary/nitrogen6x/800mhz_4x128mx16.cfg

board/boundary/nitrogen6x/800mhz_4x256mx16.cfg

board/boundary/nitrogen6x/Makefile

board/boundary/nitrogen6x/README

board/boundary/nitrogen6x/clocks.cfg

board/boundary/nitrogen6x/ddr-setup.cfg

board/boundary/nitrogen6x/nitrogen6dl.cfg

board/boundary/nitrogen6x/nitrogen6dl2g.cfg

board/boundary/nitrogen6x/nitrogen6q.cfg

board/boundary/nitrogen6x/nitrogen6q2g.cfg

board/boundary/nitrogen6x/nitrogen6s.cfg