cmake: support array of maps in yaml module

This commit introduce support for maps in a yaml list.

The yaml_set() function has been extended with the following signature:
> yaml_set(NAME <name> KEY <key>...
>          [APPEND] LIST MAP <map1> MAP <map2> MAP ...
> )

where a `MAP <map>` has the form:
`MAP "<key1>: <value1>, <key2>: <value2>, ...`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>