linux-yocto: add machine aliases for yocto BSPs

linux-yocto: add machine aliases for yocto BSPs

To avoid mapping machine names to kernel machine names in recipes,
we can define multiple KMACHINE names for a single in tree board.
This allows the tools to match a board description to multiple
different MACHINEs.

As a result, we can remove the explicit KMACHINE mappings from
the linux-yocto recipes and allow the KMACHINE=${MACHINE} default
to handle mappings. Individual recipes an bbappends can override
this as required.

(From OE-Core rev: d3dc05a4b808136876cb257a568737abfaceeadd)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>