CMake: Refactor Silicon Laboratories targets ...

Refactor all Silicon Laboratories targets to be CMake buildsystem targets. This removes
the need for checking MBED_TARGET_LABELS repeatedly and allows us to be
more flexible in the way we include MBED_TARGET source in the build.

A side effect of this is it will allow us to support custom targets
without breaking the build for 'standard' targets, as we use CMake's
standard mechanism for adding build rules to the build system, rather
than implementing our own layer of logic to exclude files not needed for
the target being built. Using this approach, if an MBED_TARGET is not
linked to using `target_link_libraries` its source files will not be
added to the build. This means custom target source can be added to the
user's application CMakeLists.txt without polluting the build system
when trying to compile for a standard MBED_TARGET.

CMake: Add support for SiLabs SL_RAIL

Silabs: Remove EFR32_1 rail files ...

Mbed OS 6 does not support Silicon Labs EFR32_1 rail

Remove EFR32MG1_BRD4150 target

Remove TB_SENSE_1 target

Revert file permissions to non-executable

Update to EFR32 15.4 driver ...

* Updates driver library to v2.3.1 (2018q1) for bugfixes and convenience functions
* Provides library in correct format (2-byte wchar_t flag) for compiling with ARMCC (#6695 uncovered by #6577)
* Reverts to using a statically-allocated packet buffer since malloc is not thread-safe (and the asserts have been turned on)

Upgrade RAIL to v2.1.1

[Silicon Labs] Rename targets ...

Due to limitation in the mbed website backend (board names need to be <= 19 characters), we are shortening the CLI target names from THUNDERBOARD to TB.
@screamerbg

Stop using device_has for non-mbed options ...

As asked by @0xc0170 in PR #3934, we won't be using device_has for indicating RF/Crypto features any longer. RF config options moved to the SL_RAIL lib.json, crypto config options will come with mbedTLS integration.

Update to Gecko SDK 5.1.2 ...

Update RAIL library to 1.5.1

First pass on TB Sense (and EFR32MG12) support

[EFR32] Move Nanostack driver to Nanostack folder ...

mbed compile doesn't support two different FEATURE_X folders being merged, so we'll have to move our nanostack driver into the Nanostack folder for the time being.

[EFR32] Prevent compiling Nanostack driver without Nanostack

[EFR32] Initial radio driver ...

First check-in of the EFR32 radio driver for Nanostack