core/pkg-virtual: do not check they are neabled

Virtual packages do not have a Kconfig symbol, so we must not check that
they are enabled.

Fixes:
    http://autobuild.buildroot.org/results/287/28713478cc6edf8e5d5c3e830fee86a42f0afa8d/


    ... and numerous similar failures ...

[Thomas:
 - use "$($(PKG)_IS_VIRTUAL)" != "YES" instead of !
   "$($(PKG)_IS_VIRTUAL)" = "YES"]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>