DeviceKey: Fix random key doesn't generate with custom entropy source

[ccli8]

Description

This PR tries to fix random key doesn't generate with custom entropy source in device_key. Originally, when DEVICE_TRNG is defined, MBEDTLS_ENTROPY_HARDWARE_ALT will also be defined accordingly to provide entropy source. This is fine for targets supporting TRNG. However, for targets without TRNG, it is also possible to provide non-TRNG entropy source solution via the define MBEDTLS_ENTROPY_HARDWARE_ALT. Related discussion can be found at #11680

Pull request type

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

[ciarmcom]

@ccli8, thank you for your changes.
@ARMmbed/mbed-os-storage @ARMmbed/mbed-os-maintainers please review.

[VeijoPesonen]

@ARMmbed/mbed-os-crypto are you fine with this?

[yanesca]

Yes, I think this should be fine:

• MBEDTLS_ENTROPY_HARDWARE_ALT is the same mechanism we register TRNGs. The user can use the same mechanism to register something as an entropy source that is weaker than a TRNG. (This might or might not be a security issue depending on the user's threat model.) This was possible before and works without a functional problem in most of the cases. This is the only function that we know of that does not work with user defined MBEDTLS_ENTROPY_HARDWARE_ALT.

• There is room for improvement regarding security, but I think that any such improvement is out of scope for this PR.

[0xc0170]

@VeijoPesonen we should be good to proceed here?

[VeijoPesonen]

Approved securitywise by @yanesca. Looks also good to me.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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