USB: Add support for STM32F446

[adbridge]

This PR adds USB support for the STM32F446

[adbridge]

/morph test

[mbed-bot]

Result: SUCCESS

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/morph test

Output

mbed Build Number: 67

All builds and test passed!

[0xc0170]

cc @bcostm @adustm @LMESTM @jeromecoutant

[adustm]

Hello @adbridge
We have thought of a new way of handling the USB support outside of the Feature library.
We would be in favor of the PR #3988 way of implementing.
Is there any reason why you implemented it another way ?
I can understand that it does not look very clear, sorry for this.

Kind regards
Armelle

[adbridge]

@adustm Hi, this was a direct port of the solution that was submitted on the mbed org site (https://developer.mbed.org/users/mbed_official/code/USBDevice/rev/7662d9ed5011). It was submitted quite sometime ago so may account for why it isn't the optimal solution now. We have been tidying up the PRs there and trying to make sure all of them are back ported here. If you'd like to do this another way, please feel free to submit an alternative. We can close this in favour of a new approach, however this is the approach currently now in the mbed 2 repo (USBDevice).
Regards, Anna

[0xc0170]

We would be in favor of the PR #3988 way of implementing.

We can close this in favour of a new approach, however this is the approach currently now in the mbed 2 repo (USBDevice).

+1, lets close this one and focus on #3988