android: clearenv should not set environ to NULL

Remove the old internal clearenv() function which is
no more required (exported in libc)

Change-Id: I357f6a3b057c366761b481c09b471d7c53d7a419
login: implement missing selinux functions

get_default_context() and security_compute_relabel()

Note: these stuff are just compatible with busybox source code
Should be enhanced, or implemented in libselinux...

Change-Id: I152be878fd1d0fa226346448200b425289279ce6