Fix Makefile error wrt spritegen

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.

https://code.launchpad.net/~bac/juju-gui/spritegen-trivial/+merge/133063

(do not edit description out of merge proposal)

[bac, 2012-11-06 13:19:11]

Please take a look.

[bac, 2012-11-06 13:21:49]

*** 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=
CC=
https://codereview.appspot.com/6814092