Support conditional rebuild

[mikee47]

As suggested in #89 this PR changes the build logic to detect when a schema has changed due to build variables.

Input .cfgdb schema is parsed and expressions evaluated. This schema is written as a .json file to out/Esp8266/debug/ConfigDB/schema.
Now, this is done as a pre-parsing stage and the .json is only updated if it is different to the existing one. This is managed by a new --preprocess flag for dbgen.py.
Standard make logic then detects the new files(s) and does a full rebuild.

Additional changes:

• Add support for conditional selection using dictionary values
• Revert output directory to out/ConfigDB

[mikee47]

makes me wonder if the DSL needs a switch statement)

I've added a commit with a suggested way to handle a switch/choose using a dict. The first key which evaluates True is chosen.

    "TestMode": {
      "type": "string",
      "ctype": "Color",
      "@enum": {
        "SMING_RELEASE": [
          "silent",
          "error"
        ],
        "1": [
          "silent",
          "error",
          "debug",
          "info"
        ]
      }
    }