Restrict target_overrides usage

[theotherjimmy]

This will prevent a user from doing the invalid:

{
   "target_overrides": {
        "*": {
            "foo": {},
            "foo.foo.foo": true
        }
}

[cmonr]

For context, why is the above pattern bad?

Is foo.foo.foo not considered its own thing?

[theotherjimmy]

@cmonr We only have a 2 level hierarchy: {libname|target|app}.parameter

[theotherjimmy]

Further, setting a configuration parameter to {} is not allowed. Both lines generate errors (at the same time)

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1050
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5998/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 731
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5998/

[mbed-ci]

Test : SUCCESS

Build number : 858
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5998/858