board/boundarydevices: rename folder and update readme

It didn't make sense to keep the U-Boot scripts and post build/image
scripts under the nitrogen6x folder since they apply for all our
platforms.

Also update the readme.txt to make it clearer which defconfig should
be used depending on the platform targeted.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>