use build/ as default coreDir

we expect the core to be installed in build/ directory. Less task will search
for dependencies in there. Copy task has to make sure, all dependencies can be
found. This is done in the Core, so every newer installation of core will work
with this configuration.
add rootpath option to less task

this forces cache-busting for each new build. Adding this to the rootpath
is fine, because appserver middlerware and apache are configured to work
with this part in the path set.