Update exec_env.py to mount /dev/null and /dev/urandom

[pjdubya]

Without this change, builds fail due to invalid reference/permissions for /dev/null and /dev/urandom. This change mounts those in the environment to eliminate those errors.

Fix https://github.com/SynologyOpenSource/pkgscripts-ng/issues/48

Note this is targeting DSM7.0 but a port is likely is needed in other branches too.