Subpackages is a feature focused on packages.
However, port builders lack the ability to install portion of a port,
causing a very different behavior.
With this patch we add the ability to install only subpackages
specificed by the TARGET_SUBPACKAGES variable.
This variable is going to contain a comma separated list of subpackages.
If the variable is missing, all subpackages are installed
The variable works for install and reinstall targets.
The deinstall targets keep the previous behavior, removing the whole port.