ubox: fix kmod loading for modules with varying dashes & underscores in names (#14182, #14189)

Instead of canonicalizing module names in the internal avl tree simpyl use a
custom strcmp() variant for the avl tree key comparisation. The custom comperator
simply treats dashes and underscores the same, so "nls_iso8859-15" is equal to
"nls-iso8859-15".

This allows us to simplify the code considerably and makes manual "modprobe"
invocations more forgiving when looking up the given module name.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38248 3c298f89-4303-0410-b956-a3cf2f4a3e73