setup-mel-builddir: pass -l layers as required, not optional

We want to fail when a user requested layer doesn't exist, to prevent later
problems, and catch typos.

JIRA: SB-1723

Signed-off-by: Christopher Larson <kergoth@gmail.com>
setup-mel-builddir: let -l args be either layer paths or names

JIRA: SB-1723

Signed-off-by: Christopher Larson <kergoth@gmail.com>