When parsing versions for the --version argument, allow a field to be
zero. Also, restrict the build to just an integer to match what we
allow (rather than allow alphabetic, and then failing to parse as an
integer).
In addition, add the missing import of argparse, so that when the
version is invalid, we get nice usage rather than an error about a
missing module.
Jira: MCUB-58
This is the start of a python implementation of imgtool. This
implements all of the functionality that was missing in the zep2newt.py
tool, namely creation of keypairs, and converting the public version of
these keys into C code.
David Brown