buildroot

configs: add Boundary Devices i.MX7 Nitrogen7 support

This board is using a different kernel and u-boot branch than the
other Boundary Devices platforms since NXP forked the 3.14 kernel to add
i.MX7 support (see imx_3.14.38_6ul7d_beta branch).
Next kernel version, 3.14.52, will be the same for all i.MX platforms.

https://boundarydevices.com/product/nitrogen7/

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