ETH: Add message signing

[NickeZ]

I need a conceptACK of this before continuing.

edit: reference for reference: https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign
prusnak reaction: ethereum/go-ethereum#14794
You can verify signature here: https://etherscan.io/verifySig

[benma]

concept ACK

[NickeZ]

@benma rereview bitte

[codecov-io]

Codecov Report

Merging #321 into master will decrease coverage by 40.34%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #321       +/-   ##
=========================================
- Coverage   40.34%     0%   -40.35%     
=========================================
  Files          97     98        +1     
  Lines        5998   6063       +65     
=========================================
- Hits         2420      0     -2420     
- Misses       3578   6063     +2485

Impacted Files	Coverage Δ
src/commander/commander_eth.c	0% <0%> (ø)	⬆️
src/apps/eth/eth_sign_msg.c	0% <0%> (ø)
src/apps/btc/btc_bip143.c	0% <0%> (-100%)	⬇️
src/workflow/cancel.c	0% <0%> (-100%)	⬇️
src/workflow/status.c	0% <0%> (-100%)	⬇️
src/apps/btc/btc_params.c	0% <0%> (-100%)	⬇️
src/apps/btc/confirm_locktime_rbf.c	0% <0%> (-100%)	⬇️
src/ui/event_handler.c	0% <0%> (-100%)	⬇️
src/salt.c	0% <0%> (-100%)	⬇️
src/apps/eth/eth_sign.c	0% <0%> (-96.56%)	⬇️
... and 41 more

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