nitrogen6x: update non-Android boot script for additional use cases

-- Added support for loading DTB. This requires the 'cpu' and 'board'
environment variables as implemented in commit 093a58639.

-- Added bootpart variable to control 'root=' command-line variable
defaults to 1 for single-partition use. Yocto generally uses '2'.

-- Requires 'allow_noncea' to enable non-CEA modes as described in
http://boundarydevices.com/hdmi/

-- Added bootdir variable to allow kernel and DTB to reside in
directories other than /boot (the default location). The current
Yocto image places the kernel in the root directory.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>