Extract EventLoggings log directory into a variable

vanadium is running low on space on the disk that EventLogging is
logging to. Since vanadium has a pretty empty mount, we prepare to
move the log files to a directory on this rather empty mount. To ease
this switch, we extract the currently hard-coded paths into a
variable.

Change-Id: If4839e99cc87c99a09e2373122678fb9b4c787eb