cros_workon: fix regression in "list-all" behavior
cros_workon: fix regression in "list-all" behavior

The addition of the mask file made the output of "list-all" a little
weird as it includes it as a "board".  Filter out all *.mask files
to avoid that.

BUG=None
TEST=`./cros_workon list-all` no longer shows "x86-alex.mask:"

Change-Id: I42c1f6efb53e1d75840c49901c6d2d9edd997fd4
Reviewed-on: https://gerrit.chromium.org/gerrit/18397
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util