Log Message:
Add a new variable to specify the installation prefix for
X11 packages (currently XFree86 and xorg), X11ROOT_PREFIX.

Defaults:

xorg: X11ROOT_PREFIX = xorg.
XFree86: X11ROOT_PREFIX = XFree86.

Otherwise it's undefined.

With this modification we don't have to specify X11BASE anymore,
because it's assigned automatically via bsd.pkg.defaults.mk.

If you want to change the defaults, specify X11ROOT_PREFIX in mk.conf.

Update Packages.txt now that we don't need X11BASE.