ucm: allow passing variables through ucm open string

ucm: allow passing variables through ucm open string

It is useful to pass information like application capabilities
to the UCM configuration parser. Those variables are prefixed
with '@' for the configuration files.

An example:

   "<<<v1='a b c',x=12>>>hw:1"

Variables can substituted in the configuration:

   "${var:@v1}" -> "a b c"
   "${var:@x}" -> 12

Signed-off-by: Jaroslav Kysela <perex@perex.cz>