Add /firmware into INSTALL_MASK.
Add /firmware into INSTALL_MASK.

We will add more firmware-related tools (static AU programs, updater, netboot
firmware, firmware symbols, firmware for AU testing, ...) into /firmware for
buildbot archiver to output, and should not be picked into target chromiumos
image rootfs; so adding INSTALL_MASK should be the right approach.

BUG=chromium-os:33338
TEST=./build_packages; ./build_image

Change-Id: I64a4477a8ca1a5ed99e32d92767509f21a77c704
Reviewed-on: https://gerrit.chromium.org/gerrit/29411
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

mjg59/scripts,nicescale/scripts,crawford/scripts,andrejro/coreos--scripts,crawford/scripts,marineam/coreos-scripts,mischief/scripts,gtank/scripts,marineam/coreos-scripts,trnubo/scripts,andrejro/coreos--scripts,bcwaldon/coreos-scripts,mjg59/scripts,bcwaldon/coreos-scripts,sigma/coreos-scripts,bcwaldon/coreos-scripts,endocode/scripts,fivethreeo/scripts,zhang0137/scripts,vmware/coreos-scripts,BugRoger/scripts,zhang0137/scripts,mjg59/scripts,vmware/coreos-scripts,endocode/scripts,coreos/scripts,gtank/scripts,BugRoger/scripts,trnubo/scripts,glevand/coreos--scripts,andrejro/coreos--scripts,sigma/coreos-scripts,sigma/coreos-scripts,fivethreeo/scripts,crawford/scripts,glevand/coreos--scripts,mischief/scripts,zhang0137/scripts,smilart/scripts,marineam/coreos-scripts,vmware/coreos-scripts,mischief/scripts,fivethreeo/scripts,BugRoger/scripts,smilart/scripts,smilart/scripts,endocode/scripts,nicescale/scripts,trnubo/scripts,gtank/scripts,coreos/scripts,nicescale/scripts