While building proxmark3 in an environment without `git` installed, I
noticed the following errors on stderr:
```
mkversion create test env: 'git': No such file or directory
stat: cannot statx '../README.md': No such file or directory
```
The first line "env: 'git': No such file or directory" is expected, as
git is not installed.
However, the second line "stat: cannot statx '../README.md': No such
file or directory" is a real error: we're running from top-level
directory, so the correct path for README.md is just "README.md".
and add --force to forcibly regenerate version_pm3.c
and small fixes:
* document mkversion.sh usage
* remove call to mkversion.sh in client/experimental_lib/CMakeLists.txt on release
* remove call to old `mkversion.pl` in bootrom/Makefile
* just in case, mkversion.sh takes care of atomic write of the file
