cmd/clef: support trezor signing

[gballet]

Hi there,

System information

Geth version: 1.9
OS & Version: Windows/Linux/OSX

Expected behaviour

To be able to sign messages with Trezor.

Actual behaviour

Approve? [y/N]:
> y
WARN [07-09|11:54:49.351] Key does not exist                       key=0xdeadbeefdeadbeefdeadbeefdeadbeef
## Password for signing

Please enter password for signing data with account 0xdeadbeefdeadbeefdeadbeefdeadbeef
> -----------------------
## Error 
not supported
-------------------------------------------

Steps to reproduce the behaviour

Using checkpoint-adming sign with a trezor-generated address.

[adamschmideg]

Closing this as it's not supported by Trezor. We can only pick it up if they implement it upstream. Closing until then.

[holiman]

Blocked by trezor/trezor-firmware#148