The comments in modutils/depmod.c suggest that option -b is supported, but this does not appear to be the case when running `busybox depmod -b / -n`. On the other hand, `busybox depmod -n` works. A comment in a commit from the upstream repository suggests that -b should work:
It looks like getopt32() is failing. Fixing this should enable easy resolution of bug #472312, which is blocking genkernel stabilization.