sys/config: Add "config delete" CLI command
sys/config: Add "config delete" CLI command

Add a "config delete" subcommand, e.g.

    config delete id/serial

Note: config groups must implement correct delete behavior (do the right
thing when a value is set to "").

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core