STM32L4: link issue with IAR

[jeromecoutant]

Summary of changes

With IAR, since #13452,
MBED_CONF_TARGET_BOOT_STACK_SIZE is not known during link.

So we need to define this symbol at the beginning of icf file to avoid any link issue.

Impact of changes

Migration actions required

Documentation

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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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Reviewers

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[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_dynamic-memory-usage	✔️
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