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>