Create a "merchant terminal" build that has none of the wallet logic

[str4d]

RUSTFLAGS='--cfg=zallet_build="merchant_terminal"' cargo build

This:

• is intended for IVK-only transaction detection, for merchant terminals.
• has no keystore.
• has no APIs for importing mnemonics, seeds, spending keys, or full viewing keys.
• has no APIs for balance (IVKs cannot correctly maintain balance).
• has no APIs for spending.
• is only tested on Linux, and the tests are not required to pass (it will not block Zallet development).
• is best-effort only.

[str4d]

Force-pushed to fix the config flag logic in the build script (which was enabling both merchant_terminal and wallet simultaneously, causing the merchant_terminal build to be identical to wallet).

[str4d]

All this could probably be made more ergonomic with the cfg-if crate.

[nuttycom]

utACK 676033d

I think I'd mildly prefer wallet to be a feature flag that is default-enabled; the machinery to automatically set the config flag seems a little janky to me. However, once we introduce IVK import, that RPC method would be visible in a "wallet" build given the additive nature of feature flags. Until we can adjust the zcash_client_sqlite implementation of APIs that return balance information to ensure that balance (which would almost certainly be incorrect) is never returned for IVK-only accounts, this approach is good.

[str4d]

Force-pushed to fix test failures.

[nuttycom]

re-utACK 5ae91d0