build: use deterministic git rev-parse

[mmilata]

No description provided.

[matejcik]

this won't help in the general case: --short=7 will show at minimum 7 chars but there's no upper limit.
we'll need something like git rev-parse HEAD then result[:7]

[matejcik]

perhaps it is better to define MICROPY_BANNER_NAME_AND_VERSION symbol to trezor or sth

[mmilata]

I see, it uses the output of git describe --tags which can also change on shallow repo with no tags. Overriding MICROPY_BANNER_NAME_AND_VERSION is better and needs no changes in this repo.