Disable write buffering on debug builds.

[SeppoTakalo]

Summary of changes

In mbed_start_application() there was a code that was supposed to
set DISDEFWBUF to one when running a debug build. However, this code
was in the wrong place, as this function is only called from
bootloader.

Move the code to correct place so that standalone applications use it
as well.

For the reference of DISDEFWBUF bit, see
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0552a/CHDCBHEE.html

Impact of changes

Debug builds should now show all HardFault locations more accurately than with buffering.
But debug builds run potentially slower as one buffering is disable. However, difference is probably theoretical.

Migration actions required

None.

Documentation

None.

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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

@kjbracey-arm

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[kjbracey]

However, difference is probably theoretical.

You could certainly measure it in a benchmark, but I'd estimate the effect is less than the loss due to compiler optimisation being off/low in a debug build - given that the RAM is normally SRAM, so relatively fast.

[jamesbeyond]

CI started

[mbed-ci]

Test run: SUCCESS

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

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

I've fixed the version: Set to 6.0.0-alpha-2