package/cryptodev: make it behave more like other virtual packages

Currently, the virtual package cryptodev is a special virtual package,
as it offers a choice for its implementation, rather than letting the
user enable them manually.

In so doing, it defines its _HAS option as a 'def_bool y' rather than
letting each implementation 'select' it.

Since we are going to generate a list of virtual packages and their
providers, this defeats the heuristic used to find providers.

Coming with an alternate heuristic that also matches the cryptodev
package is quite complex, so better and easier to make it look more
like any other virtual package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>