[yocto] meta-zephyr layer

[Bystricky, Juro]

There is interest in the community to support building Zephyr images
( https://www.zephyrproject.org/ )in Yocto via bitbake recipes. 
AFAIK the only way thus far to build Zephyr images is based on command 
line/Kconfig, similar to building Linux kernel images. 

Building of Zephyr images in Yocto can now be done fairly unobtrusively 
via a new layer "meta-zephyr" and specifying a new distro in local.conf:
DISTRO="zephyr"

The repository for the meta-zephyr is here: 
http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/log/?h=juro/meta-zephyr.

The new meta-zephyr layer contains several recipes to build (and run
in QEMUs) various Zephyr tests. There is also a sample recipe to build 
Zephyr sample "philosophers" with instructions how to run it in QEMU.

This is the first kick at the can.
At present, only x86 and ARM CortexM3 toolchains are supported, with
more coming eventually. (Zephyr also supports BSPs for ARC, Nios2, IAMCU,
and Xtensa and RISC-V coming in the next release)

The new meta-zephyr is work based on previous original work by 
Randy Witt and Richard Purdie, so it is actually a second kick at the can.

This is a work in progress, any feedback is appreciated.

Juro