toolchain-scripts: pass env to post-relocate

toolchain-scripts: pass env to post-relocate

It's useful for the post-relocate scripts to be able to see the SDK
environment, for example to see the values of CC, CXX etc. in order to
dynamically generate toolchain files.

To enable this, source the SDK environment script prior to calling the
relocate scripts.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>