Looking into my openWRT, I do not have any libnss* libraries installed. It seems that only libuClibc is used for that. libc.so.0 is a symlink to it.
root@RuiWifi:/lib# grep -ri hosts *
uClibc is an implementation of the standard C library that is much
smaller than glibc, which makes it useful for embedded systems.
If you are trying to put together a minimal environment, I would advise you to compile busybox against UClibc instead of glibc, and snooping around openWRT to see how they managed to put together such a distribution with such a small footprint.
Compiling BusyBox with uClibc