lxc-busybox: check when bind-mounting host libdirs

The patch removes the behavior of automatically mounting /lib
and /usr/lib, since this is duplicated a few lines below. It will
also remove the risk of failing when one of these entries are not
present on the host - e.g. on a 64bit machine.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>