elementary: Add PACKAGECONFIG for ewebkit and disable it by default

* with floating dependency some packages can fail to detect elementary:
  Package 'ewebkit', required by 'elementary', not found
* webkit-efl isn't in very good shape, so I don't want to force using
  it on everybody -> disable ewebkit by default

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>