LED signaling fixes

[sergeuz]

This PR introduces the following changes:

• Added support for custom LED colors in bootloader (firmware update, safe mode, DFU mode), v11.
• Combined LEDStatus and LEDCustomStatus into single class (LEDStatus). I've found it can be somewhat confusing to choose when to use which class, so the less classes the better.
• Fixed LED indication shown during device key generation (blinking white).
• Removed obsolete SPARK_LED_FADE flag.
• Added more unit/API tests.

@technobly: I'm currently finishing reference docs for the new functionality, should I submit them separately or include into this PR?

---

Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation
• Add to CHANGELOG.md after merging (add links to docs and issues)

Bugfix

• [PR #1225] [Photon/P1/Electron] Fixed LED indication shown during device key generation (blinking white) introduced in 0.6.1-rc.1

Enhancement

• [PR #1225] [Photon/P1/Electron] Combined LEDStatus and LEDCustomStatus into a single class -> LEDStatus

Feature

• [PR #1225] [Photon/P1/Electron] Added support for custom LED colors in bootloader v11 (Safe Mode, DFU Mode, Firmware Reset).

[technobly]

@sergeuz adding the docs here in this PR should be fine.

[sergeuz]

This PR should be ready for merging, once the checks are passed. The docs will go to docs/prerelease branch in a separate PR.

Electron and Photon are updating their bootloaders as expected and overall seem to work normally in my tests (tested with 0.5.3 and 0.6.1-rc.1 bootloaders). I don't have P1 to make similar test though.

[technobly]

✅ P1 sFlash tests passed.