Parse timestamp to datetime

[tsusanka]

In Stellar and most likely in some other apps as well the host is sending a timestamp to Trezor. We currently do not have a function to format it into something more readable. Since we probably (?) do not want to include the whole datetime micropython library we can either:

1. Use C and its time.h library (implemented in tsusanka/timestamp, but it fails on multiple definition of snprintf).
2. Write it manually.
3. Add the datetime library.

[prusnak]

We could take the relevant parts from datetime.py in case we want to go with option 3

[prusnak]

Another place where we could use this function is here:

trezor-firmware/core/src/apps/wallet/sign_tx/layout.py

Line 92 in 3d50bb5

text.bold(str(lock_time))

(when lock_time is interpreted as timestamp and not the blockheight).

[prusnak]

Option 2) code in C: https://stackoverflow.com/questions/20826847/convert-between-date-time-and-time-stamp-without-using-std-library-routines

[matejcik]

for QA:

Prepare a Bitcoin or Testnet transaction in Suite.
Add a locktime.
Use e.g. https://www.epochconverter.com/ to generate a timestamp in the future, use the Epoch timestamp value as locktime.

Check that the locktime confirmation screen on Trezor is showing the right date & time.

The transaction does not need to be sent, this is purely a display change.

[bosomt]

QA OK

fw trezor-fw-regular-2.4.3-221977ad.bin
fw trezor-fw-regular-1.10.4-221977ad.bin