Fix remaining bionic warnings

Tested on arm and x86 (kitkat),
aosp/master has some other ones

Also update the readme and android.h

Change-Id: I170b16f2e091b99f971fb518bf0106c0e92adfdd
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 maybe not be merged as-is later (new applets)

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
Android.mk: toggle l-preview config flag

Change-Id: I4465b88a0337f5f267d501a94b2f71f8ef07b318
android: fix building errors with Android L

Tested OK with Android L PDK LRW17D.

The asm files are regenerated by Android L gensyscalls.py.
syscalls: restore compatibilty with current branch

on arm __NR_stime constant is undefined in asm/unistd.h

add also BIONIC_L define for assembly files (syscalls)
tested on arm and x86 (not on mips)

Change-Id: If5b6a6d3f8b4c10339f8e7b0bf340d02c1cf26cb
Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>