init: get rid of the remaining double mounts

Don't double mount /dev and its subdirectories anymore. Instead, the
first stage init is solely responsible for mounting it.

Don't have init prepare the property space. This is the responsibility
of the second stage init.

Don't have SELinux use the property space to determine how we should
be running. Instead, create a new function and extract the data we
need directly from /proc/cmdline. SELinux needs this information in
the first stage init process where the property service isn't available.

Change-Id: I5b4f3bec79463a7381a68f30bdda78b5cc122a96