IOTBTOOL-377: Fix scatter file include path online

[mark-edgeworth]

Description

Fixes an online IDE problem that was preventing any #include paths inside scatter files from being resolved

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

See the extensive comment in this change for a full description of how this works (and why it didn't)

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)

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

This change is a back-ported change which is already in place in the online IDE.

[ciarmcom]

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

[0xc0170]

Interesting findings how the online version differs!

[0xc0170]

Ci started meanwhile

[mbed-ci]

Test run: FAILED

Summary: 3 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-test
• jenkins-ci/mbed-os-ci_dynamic-memory-usage
• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

The job was restarted, thus failure above

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts

[0xc0170]

@mark-edgeworth Expecting anyone else from @ARMmbed/mbed-os-tools to review this one?

[mark-edgeworth]

Probably not; Graham might be interested but he's on vacation until Monday.

[0xc0170]

Restarted travis to report the status again