Fix build failure when target's cmsis doesn't exist

[ccli8]

Summary of changes

For bootloader enabled target, it needs to have cmsis pack or provide memory override to build successfully. This PR tries to fix build failure on targets having no cmsis pack but providing memory override with start being zero. Start address of zero is valid but it is misjudged as no memory override.

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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

[ciarmcom]

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

[0xc0170]

CI started

[0xc0170]

I restarted CI, did not propagate results

[mbed-ci]

Test run: FAILED

Summary: 4 of 11 test jobs failed
Build number : 2
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_mbed2-build-GCC_ARM
• jenkins-ci/mbed-os-ci_mbed2-build-IAR
• jenkins-ci/mbed-os-ci_mbed2-build-ARM
• jenkins-ci/mbed-os-ci_cloud-client-pytest

[0xc0170]

Client restarted, the rest looks fine

[0xc0170]

@ARMmbed/mbed-os-tools Please review, if approved, this can go in