Boot: Use the PyPI version of imgtool

Modifies the Python wrapper for imgtool in TF-M to use the PyPI
version instead of the one in the MCUBoot repository.

Signed-off-by: Balint Matyi <Balint.Matyi@arm.com>
Change-Id: I0b3fb9c802c59628cced0962241a1b4573a00cfc
4 files changed