package/libssh: add support for mbedtls crypto backend

package/libssh: add support for mbedtls crypto backend

At this point Buildroot doesn't allow to use mbedTLS crypto
backend even though libssh supports it. In case of fully statically
linked ELF executables the size difference between OpenSSL and mbedTLS
is significant: it matters for embedded targets with very limited
storage.

This patch adds support for compiling libssh with mbedTLS as a crypto
backend.  It also allows the selection of the crypto backend libssh will use
through a choice in the package config, similar to libssh2.

Currently, the selection of the backend is based on a priority order,
which is not always desirable, as in some cases multiple backends
can exists at the same time for various reasons.

Signed-off-by: Mircea Gliga <gliga.mircea@gmail.com>
[Peter: use depends on rather can select for consistency with libssh2]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>