Previously, devmatch would stop at the first linker.hints file
found in kern.module_path. This meant modules installed in
/boot/modules/ were invisible to devmatch if /boot/kernel/
contained a linker.hints file (which it always does).

Merge hints from all directories in kern.module_path.
This allows third-party or out-of-tree kernel modules in
/boot/modules/ to be auto-loaded by devmatch just like
built-in modules.