blobmsg: add support for double

This adds support for double floating point type to make it more JSON
compatible. For type checking it also adds a stub BLOB_ATTR_DOUBLE type.
If necessary, the accessor functions for blob can be added later

Signed-off-by: André Gaul <andre@gaul.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>