libssh2: version bump

libssh2: version bump

- Bump version to 1.8.0
- Switch from openssl crypto backend to mbedtls (the package is a lot
  smaller size-wise compared to openssl and libgcrypt)
- mbedtls support was added in 1.8.0 release. Unfortunately the detection
  doesn't work out of the box, so a patch is needed that fixes an m4
  script. For that reason autoreconf must be run.
- Add --with-libz-prefix as without it zlib is not detected (currently
  there is the zlib dependency but libssh2 never actually links to it).
- Add --disable-silent-rules to get verbose build output.

Signed-off-by: Sebastian Kemper <redacted>