* [linux] Add initial Linux platform DeviceLayer.

- Adds Device Layer for Linux.
- Uses SystemLayer Sockets.
- Entropy and time implementations.
- Stub out ble, connectivity, and configuration / storage for follow-on PR.
- Uses pthreads for mutex and tasks.
- Uses std::chrono for monotonic system time.

* [linux] Add PlatformMgr tests.

* [linux] Add CircleCI target for linux device layer.

* [linux] Add fixme reference to LWIP ifdef.

* Restyled by clang-format

* [linux] fix header order; fix build

After the latest rebase, the following error was coming up:

```
../../src/include/platform/ConnectivityManager.h:252:40: fatal error: platform/CHIP_DEVICE_LAYER_TARGET/ConnectivityManagerImpl.h: No such file or directory
  252 | #define CONNECTIVITYMANAGERIMPL_HEADER <platform/CHIP_DEVICE_LAYER_TARGET/ConnectivityManagerImpl.h>
```

Though CHIP_DEVICE_LAYER_TARGET=Linux is set using AC_SUBST in configure.ac,
the definition, which was working fine before, was mysteriously getting lost.
It turns out the header re-ordering PR was the root cause of the issue.

The fix was to change all sources in src/platform/Linux to
move this include first:
    #include <platform/internal/CHIPDeviceLayerInternal.h>

The error did not lead to the fix in an obvious way, so the issue
is being documented here.

* [linux] Update linux-embedded target to follow circleci matrix template.

* [linux] Fix naming nits.

Co-authored-by: Restyled.io <commits@restyled.io>