nitrogen6x: add meta-boundary and create boundary group

Source code of the meta can be found here:
https://github.com/boundarydevices/meta-boundary

This meta layer is especially useful since our boards now ship with the
BD-SDMAC WiFi/BT module:
https://boundarydevices.com/product/bd_sdmac_wifi/

This latter requires a specific external kernel module and a modified
version of BlueZ hciattach tool.

BD-SDMAC support is limited to WiFi since BT firmware is only available
upon agreement to Silex EULA by registering your SSH key to our website:
https://boundarydevices.com/private-git-access/



Even for our older TIWI-BLE module, this layer is necessary to include
the BT firmware files that are currently missing (TIInit_7.6.15.bts).

Change-Id: I00560fb016a095c6d98c8031ac9c1873c37f71a5
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>