packages: add infrastructure for virtual packages

The virtual-package infrastructure allows to easily define a
virtual package in a single line:

    $ cat package/some-virtual-package/some-virtual-package.mk
    $(eval $(virtual-package))

And that's all. :-)

Thanks to Éric for his work on the manual, that prompted the
idea for this virtual-package infrastructure! ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Mike Zick <minimod@morethan.org>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>