KVStore: Fix buffer overrun when device key size doesn't match

[ccli8]

Summary of changes

This PR tries to fix #12822, where buffer may overrun when injected device key is 32-byte but read as 16-byte.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 6 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

There are lot of failed related test, please review

[ccli8]

This PR adds check for device key size and will catch key size mismatch cases, which are missed before. #12823 can have similar issues. To address them, I make the modifications:

1. Chery-pick Allow Devicekey::generate_root_of_trust() to define key size. #12823. This enables specifying key size in generate_root_of_trust(...) and default key size changes from 32 bytes to 16 bytes to match most known code, like:

   mbed-os/features/storage/kvstore/securestore/SecureStore.cpp

   Line 93 in 97a380f

   int os_ret = devkey.generate_derived_key(salt_buf, strlen(salt), encrypt_key, DEVICE_KEY_16BYTE);

   And:

   mbed-os/features/storage/kvstore/securestore/SecureStore.cpp

   Line 125 in 97a380f

   int os_ret = devkey.generate_derived_key(salt_buf, strlen(salt), auth_key, DEVICE_KEY_16BYTE);

2. Fix test code to specify key size explicitly in the generate_root_of_trust(...) call when necessary.

Pull request has been modified.

[ccli8]

Do rebase to base on #12823

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 2
Build artifacts

[0xc0170]

Will be merged as soon as we have 6.0.0 beta1 released