FlashIAP: Get erase value from HAL instead of hardcoding it

[mnaberez]

Description (required)

Summary of change (What the change is for and why)

When programming flash, if the chunk size to program is less than the page size, the buffer is padded with the erase value. The erase value is hardcoded as 0xFF. However, since #8589, the HAL provides a flash_get_erase_value() function.

This PR replaces the hardcoded 0xFF with the value returned by flash_get_erase_value(). That value will be 0xFF for all existing targets. Removing the hardcoded assumption of 0xFF allows for MBed to support a target with a different erase value, which is the spirit of #8589.

Documentation (Details of any document updates required)

None.

---

Pull request type (required)

[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)

---

Test results (required)

[] 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]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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