Code review comment for lp:~bac/juju-gui/spritegen-trivial

*** Submitted:

Fix Makefile error wrt spritegen

If app/assets/sprite exists then regenerating sprites failed since the
logic assumed the directory would not be there. Remove the directory
before copying the contents of the newly generated sprite data.

Self-reviewed.

R=
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https://codereview.appspot.com/6814092

https://codereview.appspot.com/6814092/