Create cashew log directory from pre-start stanza in upstart script.

BUG=chromium-os:5253
TEST=Stop cashew (sudo /sbin/stop cashew). Delete log directory (sudo rm -rf /var/log/cashew). Start cashew (sudo /sbin/start cashew). Observe that log directory has been created. Restart cashew (sudo /sbin/restart cashew). Observe that existing log directory has not been disrupted.

Review URL: http://codereview.chromium.org/3036032

src/cashew.conf

diff --git a/src/cashew.conf b/src/cashew.conf
index c1d3001..f01ed90 100644
--- a/src/cashew.conf
+++ b/src/cashew.conf
@@ -4,7 +4,10 @@
 
 # cashew upstart job
 
+env CASHEW_LOGDIR=/var/log/cashew
+
 start on started flimflam
 stop on starting halt or starting reboot
 respawn
-exec /usr/sbin/cashewd --log_dir=/var/log/cashew --logbuflevel=-1
+pre-start exec mkdir -p -m 0755 $CASHEW_LOGDIR
+exec /usr/sbin/cashewd --log_dir=$CASHEW_LOGDIR --logbuflevel=-1