hd-seed: generate proper extended private key version byte when using testnet.

Achieved by using a new configuration variable `testnet` in sx.cfg, can
be set to "true" or "false" (string).

Drawback: need to include libobelisk_LIBS when building.
hd-seed: narrow build dependency to libconfig++.
config: add `testnet`, default to `false`.
Merge pull request #75 from veox/hd-seed-valid-testnet

hd-seed: proper version bytes for testnet