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
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]

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