pkg-virtual: simplify definition of FOO_VERSION to 'virtual'

pkg-virtual: simplify definition of FOO_VERSION to 'virtual'

As mentioned in the e-mail accompanying the introduction of the pkg-virtual
infrastructure [1], the definition of FOO_VERSION is 'strange'.

After the cleanup of single/double dollar signs in inner-generic-package,
the special construction in pkg-virtual is no longer needed and can be
simplified.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[1] http://lists.busybox.net/pipermail/buildroot/2014-April/093670.html
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>