Recursive search CLIXON_YANG_DIR

[pheller]

It would be convenient if Clixon recursively searched each specified CLIXON_YANG_DIR for required yang modules. The particular use case would be for OpenConfig which has a relatively broad directory hierarchy.

Not a problem to combine them in a single directory, or enumerate the list of directories with multiple instances of CLIXON_YANG_DIR, just would be more concise to use the directory as cloned from upstream and only add one configuration line to search it.

[olofhagsand]

This should work now @pheller, please verify it works as intended.

[pheller]

Looks good, thanks.