Dynamic root: Build /bin and /sbin dynamically
* Introduce a NO_DYNAMICROOT variable for people wanting to keep
fully static /bin and /sbin directories
* Keep building /sbin/init statically
* Keep a few other binaries static to avoid moving too many libraries
to /lib and/or creating many symlinks.
Those few binaries do not handle user names and thus don't need to be
NSS aware/dynamically linked.
* In particular, keeping udevd static allow us to avoid moving libpthread
and its associated links to libc_r or libthread_xu