android: on prepare, clean all intermediates

This was requested to me to prevent no clobbered builds failures
on busybox config changes.

Change-Id: I1bdf2948e0669b1de567381eb809f576e8f1eef5
mktemp: include libc mktemp, safe in busybox

This commit remove the last warning in busybox,
and prevent users to try to use mkstemp which has not
the right behavior.

Change-Id: I5aaae8044769bb1893f2430287d72b3abbee796c
Test some tty related applets (getty, login, su)

And passwd, mkpasswd, cryptpw

Sample /etc/passwd required before 'passwd' :
root:x:0:0:root:/:/system/xbin/bash

busybox su allow to change current user, if you are root
Example : "busybox su shell -c /system/xbin/bash"

busybox su is not symlinked to prevent problems with
the optional su daemon

getpwXXX() return now an empty string instead of null,
busybox often check directly pw->pw_passwd[0]

android: clearenv should not set environ to NULL

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

login: implement missing selinux functions

get_default_context() and security_compute_relabel()

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

PS: This commit was made to test applets on bionic,
and will not be merged as-is later (new applets)

I will extract some changes in a separate commit

Change-Id: I96ee64f0872856fad6b3ff299faafad6451da556
cron: fix busybox folder config

config folder was badly set to debian style cron.d
but busybox variant is a bit different :

    mkdir /etc/crontabs
    echo "* * * * * log CRON!" > /etc/crontabs/`whoami`

    crond -f

Change-Id: I0b9af08a557a952b5074da3231f32433f447f558