setup-mel-builddir: add cmdline arg for extra layers

New argument: -l "SPACE_SEPARATED LIST_OF_LAYER_NAMES"

JIRA: SB-1567

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #63 from kergoth/setup-env-layers-argument

setup-mel-builddir: add cmdline arg for extra layers