Android.mk: prevent warnings on make prepare

Add "+" to allow a new job

touch import_includes to prevent problems with -jN

Change-Id: Iea3cd527842a8bd770ff56fce01bcd0ff2d0db83
selinux: implement missing selinux functions

checkPasswdAccess() is deprecated
http://linux.die.net/man/3/selinux_check_passwd_access

also, ensure context is initialized in run_shell()

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

Change-Id: I22ea2679191d5c0887b6ef8c77f09a032c226876
Android: update busybox def PATH and cron folder

remove /usr/bin, add /vendor/bin, like official path

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
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