Support signing transactions with output amount > input amount

[kewde]

PSBTs with custom SIGHASH flags may not always have all the inputs already because we don't know who the counterparty will be yet.
This leads to a situation where the output amount > input amount and the firmware throws an error.

workaround is to add a dummy input - just make sure it's not included in the sighash of the real inputs you wish to sign.
maybe skip the amount check if all inputs have sighashes that support adding more inputs later on? they already get hit with a scary warning about custom sighashes anyways