oe-buildenv-internal: Only add to $PATH if needed

oe-buildenv-internal: Only add to $PATH if needed

First strip $PATH of any existence of the paths needed by Open Embedded
and BitBake. Then add the needed paths at the beginning. This makes sure
the needed paths are searched first, without growing $PATH unnecessarily
if oe-init-build-env is rerun for a directory for which it has
previously been run.

(From OE-Core rev: 7429db6f38e405774ba66b3fa1bc3ac4b74ae6b9)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>