kvargs: new function to get from key and value

kvargs: new function to get from key and value

A quite common scenario with kvargs is to lookup for a <key>=<value> in
a kvlist. For instance, check if name=foo is present in
name=toto,name=foo,name=bar. This is currently done in drivers/bus with
rte_kvargs_process() + the rte_kvargs_strcmp() handler.

This approach is not straightforward, and can be replaced by this new
function.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>