Correct post-build-hook detection logic

[theotherjimmy]

Description

Fixes #10032

The problem was that post build hook detection logic expected two things:

• The toolchain argument was a toolchain object
• That it could recreate toolchain labels itself

Neither of these assupmtions are correct.

I changed the logic to take in a list of toolchain labels, and delegated
the creating of toolchain labels to the toolchain. Then I cleaned up all
the pylint warnings.

Pull request type

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

[ciarmcom]

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

[theotherjimmy]

@cmonr Does travis need a restart?

[0xc0170]

@cmonr Does travis need a restart?

Travis restarted

[kimlep01]

Tested and I can confirm that this fixes both MTS_MDOT_F411RE and MTB_MTS_DRAGONFLY issues, thanks Jimmy.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 9 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-ARMC6

[0xc0170]

CI aborted, will be restarted after rc3 jobs

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 13 of 13 test jobs passed
Build number : 2
Build artifacts