package/libcurl: add option for no SSL/TLS support

package/libcurl: add option for no SSL/TLS support

Since we already have a choice to select the backend to do crypto, push
the limits even further and add an option to do no crypto.

Usually, we would have added that option first in the choice, but if we
were to do that now, existing defconfigs that previously used openssl
(the first item in the choice) would now default to non crypto, which is
not so nice. So we add the new option last in the choice.

Each crypto backend option is used in a conditional block, each of which
default to disabling said backend. So, selecting none will indeed
disable all.

We can now drop the blind intermediate option that would hide the choice
when no backend library was available; there will now always be at least
the none option in the choice, so we need not hide it.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Julien Grossholtz <julien.grossholtz@openest.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>