cros_setup_toolchains: add an "sdk" target to stop overloading "all"
cros_setup_toolchains: add an "sdk" target to stop overloading "all"

In some places, we're using "all" to mean "all sdk targets", but in
others, we're using it to mean "all" targets.  Rather than overload
the "all" target more, add a new special "sdk" target.

BUG=None
TEST=`cros_setup_toolchains --show-board-cfg=sdk` shows just sdk targets
TEST=`cros_setup_toolchains --show-board-cfg=all` shows all targets

Change-Id: I945fa79d09ce58818dc69c9f43b81ab4a0f7baf4
Reviewed-on: https://gerrit.chromium.org/gerrit/25625
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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