init: add support for mknod builtin

Takes the following arguments:
mknod name, type[b/c/u/p], mode, major, minor

Based on implementation from:
https://gitorious.org/openaos-cyanogenmod/device_archos/commit/79da8a8

Change-Id: Id39e5483d9cdd3404c1d052a360830821067eb34