blobmsg_json: add new functions blobmsg_format_json_value*

blobmsg_json: add new functions blobmsg_format_json_value*

The current blobmsg_format_json* functions will return invalid JSON when
the "list" argument is given as false (blobmsg_format_element() will
output the name of the blob_attr as if the value is printed as part of a
JSON object).

To avoid breaking software relying on this behaviour, introduce new
functions which will never print the blob_attr name and thus always
produce valid JSON.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cosmetic style fix]