revamp -sysroot and -hostprefix handling

instead of being a variable added to the makespec (via qconfig.pri),
QT_SYSROOT is now a property.

the QT_INSTALL_... properties are now automatically prefixed with the
sysroot; the raw values are available as QT_RAW_INSTALL_... - this is
expected to cause the least migration effort for existing projects.

-hostprefix and the new -hostbindir & -hostdatadir now feed the new
QT_HOST_... properties.

adapted the qmake feature files and the qtbase build system accordingly.

Change-Id: Iaa9b65bc10d9fe9c4988d620c70a8ce72177f8d4
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>