libssh2: revert to previous crypto defaults

The recent crypto handling change (commit 04a1031d: package/libssh2: Add
selectable crypto libraries) had the unfortunate side effect that it no
longer automatically selects the most suitable crypto backend (E.G.  one
where the dependency is already enabled), so all users not wanting to use
the mbedtls backend need to explicitly configure this.

Fix this by inverting the logic so the crypto backend sub options use
'depends on' their dependencies instead of 'select', so only the available
backends are displayed.

Like before, default to openssl if no crypto backend dependencies are
currently enabled.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>