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
Dynamic root: Move required libraries to /lib

* Add /lib to the standard run-time loader library search path

* Ensure libxxx.so symbolic links stay in /usr/lib, the build
  tools won't pick them up in /lib