Restore Ledger functionality with testnet builds

[markmhendrickson]

Currently the integration is incorrect given the compatibility patch made to the Stacks app for Ledger.

[kyranjamie]

This will require corporation with Zondax to expand the APIs exposed for the device, so mainnet & testnet addresses can be derived from the same derivation path.

[markmhendrickson]

Have you given them a heads up already / should we create a related issue in the Ledger app repo for them?

[kyranjamie]

@markmhx logged here Zondax/ledger-stacks#52