Add some type checking of configs

[theotherjimmy]

Before, the types were not checked and just expected. The old behavior
would cause lots of tracebacks, or, much worse, convert things like:

{
     "target_overrides": {
        "*": {
		"target.macros_add": "CONFIG_GPIO_AS_PINRESET"
        }
     }
}

into a definition of each of the letters as macros that expand to
nothing, causing massive compilation problems.

I resolved this by adding some type checking to the config data. Now
there is a type check for most of the fields within a library and
application configurations.

resolves #3985

Testing

• /morph test

[theotherjimmy]

@jaustin Be advised. More weird type-related fixes welcome.
@bridadan is that a valid test?

[theotherjimmy]

I got PR 4000 🎉 🎈 🍰

[0xc0170]

Add some type checking of configs #4000

🍾

[0xc0170]

@bridadan is that a valid test?

tools/tests/config or config_test

[bridadan]

@theotherjimmy Please just use the regular morph test, it includes the example build step already.

[bridadan]

This looks great!

[adbridge]

/morph test

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1728

All builds and test passed!