setup-mel-builddir: ensure our BB_ENV_EXTRAWHITE changes aren't lost

We were setting BB_ENV_EXTRAWHITE before sourcing the oe-core setup script,
but it seems the oe-core setup script sets it, rather than appending to it, so
it was blowing away our additions. Add to it after sourcing the oe-core setup
script instead.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #140 from kergoth/sb-2612

setup-mel-builddir: ensure our BB_ENV_EXTRAWHITE changes aren't lost