Change imgtool lib import to relative import

[JuhPuur]

Description

Building for ARM_MUSCA_A1_NS with python 3

python tools/psa/release.py -m ARM_MUSCA_A1_S
python -m mbed test --compile -j 8 -m ARM_MUSCA_A1_NS -t ARM --build-data BUILD/tests/build_data.json -DMBED_HEAP_STATS_ENABLED=1 -DMBED_STACK_STATS_ENABLED=1 -DMBED_TRAP_ERRORS_ENABLED=1 -DMBED_ALL_STATS_ENABLED -DSKIP_TIME_DRIFT_TESTS=1

shows that imgtool has an import problem.

  File "/home/juhpuu01/build_test/mbed-os/tools/psa/tfm/bin_utils/imgtool.py", line 22, in <module>
    from imgtool_lib import keys
ModuleNotFoundError: No module named 'imgtool_lib'

This is easily fixed by changing to a relative import.
Tested to be compatible for python 2.7.15 and 3.6.8

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

Release Notes

[ciarmcom]

@JuhPuur, thank you for your changes.
@ARMmbed/mbed-os-tools @ARMmbed/mbed-os-maintainers please review.

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[OPpuolitaival]

Looks good

[OPpuolitaival]

@adbridge could we merge this?