example-ledflasher: convert to using cfgtree.mk

This change converts ledflasher to using the
cfgtree.mk-based product definition changes.

It depends on the cfgtree change and changes to
both crash_reporter and brillo_config to pull
in those two variables where expected.

This path may now be copied verbatim for out
of tree use.  The only missing convenience piece
is the envsetup.sh which a call to
  brunch product envsetup --generate
will fix.

BUG=25343470
TEST=full in-tree build in progress. an out of tree validating build will follow.

(cherry picked from commit c692b500)

Change-Id: I5efb5fadf3f6034232886d39e40880e7604e4826