cbuildbot: do not run signer tests for firmware targets
cbuildbot: do not run signer tests for firmware targets

Signer tests only work on recovery images (currently), and since firmware
targets no longer produce a recovery image, we can't run the tests.  So
disable them.

BUG=chromium-os:19543
TEST=`cbuildbot link-firmware` skips the signer tests stage

Change-Id: Ifaf776d2ae05bdbee6ae58429fdaeea189efd2a4
Reviewed-on: https://gerrit.chromium.org/gerrit/35333
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreos/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite