Change mbed-os-example-wifi to build only DISCO_L475VG_IOT01A

[jussisip]

Description (required)

Currently mbed-os-example-blinky and mbed-os-example-wifi are built for each and every target in Mbed OS. This will change that only DISCO_L475VG_IOT01A is built for mbed-os-example-wifi. DISCO_L475VG_IOT01A was chosen as its one of the wi-fi supported boards and having integrated wi-fi.

Documentation (Details of any document updates required)

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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)

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Test results (required)

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

Build passed with GCC_ARM locally in Mac OS X

Passed Example Compilation:
+----------------------+---------------------+-----------+----------+--------------+
| EXAMPLE NAME | TARGET | TOOLCHAIN | TEST GEN | BUILD RESULT |
+----------------------+---------------------+-----------+----------+--------------+
| mbed-os-example-wifi | DISCO_L475VG_IOT01A | GCC_ARM | TEST_OFF | PASSED |
+----------------------+---------------------+-----------+----------+--------------+

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Reviewers (optional)

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

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

[0xc0170]

Please add the reason in the commit message (already in PR itself). otherwise looks fine

[mark-edgeworth]

Looks ok to me

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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

[adbridge]

This is needed for 5.14 branch as wifi building there is broken