Fix NameError when arch is unsupported
Fix NameError when arch is unsupported

BUG=chromium-os:21358
TEST=try running cros_sdk on an unsupported arch (i686)

Change-Id: Idbee4d75d6fef07ea3ef98b9e58058cfed767b37
Reviewed-on: http://gerrit.chromium.org/gerrit/8916
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

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