qt4: allow recipes building commercial edition

qt4: allow recipes building commercial edition

To enable building the commercial edition of Qt (through additional
recipes that are *not* provided by OE-Core) we need to tweak a few
things:

* Don't make recipes that inherit qt4x11.bbclass depend on qt4-x11-free
  - instead add qt4-x11 to DEPENDS and then have qt4-x11-free include
  this in its PROVIDES. A commercial equivalent recipe should do the
  same.
* Add a QT_LICENSE_FILE variable that can be used to specify the license
  file required by the commercial edition.
* Add a QT_LICENSE_FLAGS variable which the recipe can set to select the
  license option being used. The default of "-opensource" retains the
  current behaviour; a commercial recipe should set it to "-commercial".

Fixes [YOCTO #2505].

(From OE-Core rev: 2be0058fc4acddab611637656183accd052b40eb)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>