fix Makefile for ebuild per platform

As mttools is required when the regression test is running as a
BVT test, the patch tries to install the replay utilities into
touchpad-tests framework for emerge target instead.

BUG=chromium-os:37644
TEST=in chroot, the command 'sudo make install' should make no
  difference with the patch. For emerge on a target board,
  try the following steps:
    1. setup_board --board=lumpy
    2. cros_workon --board=lumpy start mttools
    3. emerge-lumpy mttools
  And check if the mtlib and mtedit under mttools are built and
are installed under
    /build/lumpy/usr/local/autotest/client/deps/touchpad-tests/framework

Change-Id: Id7bf6bfa9d296dd77fb019c41914505063b1762d
Reviewed-on: https://gerrit.chromium.org/gerrit/44025
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Joseph Shyh-In Hwang <josephsih@chromium.org>
Commit-Queue: Chung-yih Wang <cywang@chromium.org>
Tested-by: Chung-yih Wang <cywang@chromium.org>