bitbake: bitbake-worker: ensure BUILDNAME is available during execution

bitbake: bitbake-worker: ensure BUILDNAME is available during execution

BUILDNAME is set from cooker by default, so since the worker split it
will not be set when executing functions. In OpenEmbedded this results
in /etc/version (which is populated from BUILDNAME) not having any
content. Pass this variable value through to the worker explicitly to
fix the issue.

Fixes [YOCTO #4818].

(Bitbake rev: 92940b0427d9b2b3f95e27c230ec1e36638a34bc)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>