Fix build errors for bare metal greentea tests on NRF52840_DK

[evedon]

Summary of changes

CI nightly tests are not building on NRF52840_DK with the following errors:

/usr/local/Cellar/arm-none-eabi-gcc/8-2018-q4-major/gcc/bin/../lib/gcc/arm-none-eabi/8.2.1/../../../../arm-none-eabi/bin/ld: 
BUILD/tests/NRF52840_DK/GCC_ARM/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/USBPhy_Nordic.o: in function `get_usb_phy()':
./targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/USBPhy_Nordic.cpp:53: multiple definition of `get_usb_phy()'; BUILD/tests/NRF52840_DK/GCC_ARM/hal/usb/mbed_usb_phy.o:
./hal/usb/mbed_usb_phy.cpp:25: first defined here

The solution is to fix the target override in baremetal.json which was wrongly removing USBDEVICE (similarly for EMAC).

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

Covered by CI nightly tests.

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Reviewers

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[ciarmcom]

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

[0xc0170]

@rajkan01 Please fix your editor to remove trailing spaces by default

[0xc0170]

@evedon I set this for 5.15.0rc2 ?

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 1
Build artifacts

[evedon]

Discussed with @jamesbeyond We can put it in 5.15 to allow user to manually run greentea tests in bare metal mode for this target.

[0xc0170]

@jamesbeyond @rajkan01 Please review

[jamesbeyond]

LGTM