Correctly process APPLICATION/SYSTEM DESCRIBE requests

[avtolstoy]

Fixes #1181

NOTE: Device currently receives incorrect DESCRIBE request with flags=0x09 (see https://github.com/spark/firmware/blob/dfc5e68866cdd794d17ab51640ca565ae3a757ab/communication/src/protocol_defs.h#L91). The length of the request also seems to be wrong (judging by #1181), and instead of 9 bytes, the device receives 16 bytes, all extra bytes are 0x09:

Photon:                        41016b7d02b16409 09 09090909090909
Electron DESCRIBE_SYSTEM:      4101305d01b16441 01
Electron DESCRIBE_APPLICATION: 4101305e02b16441 02

The PR accounts for this by assuming that the server actually requests DESCRIBE_ALL (0x03), if describe flags byte >= DESCRIBE_ALL (0x03)

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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 (N/A)
• Add to CHANGELOG.md after merging (add links to docs and issues)

Bugfix

• [PR #1226] [Fixes #1181] [Photon/P1/Core] Process TCP DESCRIBE properly and return only one response, SYSTEM, APPLICATION, or COMBINED (ALL) describe message. Was sending separate SYSTEM and APPLICATION previously.

[brycekahle]

I confirmed the TCP server does not set any flags.

The extra bytes sound like AES padding, which will always be applied to get to multiples of 16 bytes. It appears that whatever handles the AES decryption isn't adjusting the length appropriately.