cros flash: add an option to clear devserver cache
cros flash: add an option to clear devserver cache

Add --clear-cache to clear the devserver cache.

Also change --src-image to --src-image-to-delta to avoid confusion.

BUG=chromium:359385, chromium:357873
TEST=`cros flash --clear-cache`

Change-Id: I725b8a5a42ae07343941ee50da1b7dff0320bf57
Reviewed-on: https://chromium-review.googlesource.com/193062
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

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