Move the dist-as-droid handling from make to ninja

dist is a strange dual-use target, as both a flag and a goal.  make dist
<target> means build <target>, but copy any dist-for-goals files to
$DIST_DIR.  make dist without a target means build droid with
dist-for-goals files.  This requires special handling of MAKECMDGOALS to
detect the case where dist is the only goal, which breaks when ninja.mk
doesn't pass all the goals to kati.  Instead, let ninja handle the
default goal.  Pass dist to kati if it is specified in order to turn on
the dist-for-goal rules, but filter it out of the goals passed to
ninja.  If there are any other goals, ninja will build them.  If there
are no goals, ninja will fall back to the default goal, which is droid.

Change-Id: Iafb184d28cb312cb7c5682cd3ff4f310b2d9f7e9