setup-environment: don't run post-setup-env if we failed to set up BUILDDIR

Signed-off-by: Christopher Larson <kergoth@gmail.com>
setup-mel-builddir: fix automatic MACHINE determination

JIRA: SB-2878

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #189 from kergoth/sb-2878

Fix MACHINE determination in the setup scripts