toolbox/modprobe: Fallback to /lib/modules/<uname>

Make the module directory optional by reading the kernel release
version. This path is where the kernel installs modules by default.
Similar behaviour can be found in several modprobe implementations.

Bug: 254835242
Change-Id: I61707636705e5b4d9bd8ccf6351e7057eae6bcf5
1 file changed