[electrum] Add support for an eCash firmware on Trezor One

Summary:
As per title.

Because of the absence of a vendor name in the legacy firmware header, there is an extra space in the update progress window that is fixed in this diff.

Test Plan: Try it on the device

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17852