cros_setup_toolchains: support "all" with --show-board-cfg
cros_setup_toolchains: support "all" with --show-board-cfg

We have some tools that want to query all the available tuples,
so add support for doing that.

BUG=None
TEST=`cros_setup_toolchains --show-board-cfg=all` shows all tuples from all boards

Change-Id: I4d24f3a8c9bf3d16ade18f25c02a28aaeb50825f
Reviewed-on: https://gerrit.chromium.org/gerrit/24332
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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