[yocto] New software layer for the barebox bootloader

[Dennis Menschel]

Hi all!

I have created a new software layer named meta-barebox that provides
support for the barebox bootloader.

The layer has already been added to the OpenEmbedded Layer Index:
https://layers.openembedded.org/layerindex/branch/master/layer/meta-barebox/

The layer features its own reference manual that describes the interface
of meta-barebox and provides a usage example.
Last but not least, it also answers the question why I have created a
separate layer for the barebox bootloader.
The reference manual can be found here:
https://github.com/menschel-d/meta-barebox/blob/master/doc/ref-manual.md

Have fun!

Best regards,
Dennis Menschel