Android: final cleanup of new build system

Rename hidden config files to busybox-<profile>.config

Set also KBUILD_OUTPUT to prevent generation of files in src dir,
if local make without 0= is made (which create host code)

Do not force the rebuild of config files
  Check if -B parameter is present (mm -B) and do a clean
  build only if required, or if config files are not there

With the help of Chih-Wei Huang <cwhuang@linux.org.tw>

Change-Id: Ic6bc2bc7177b1f266b9c0d1ccdfb925c1aa00d46
Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>
Android.mk: toggle l-preview config flag

Change-Id: I4465b88a0337f5f267d501a94b2f71f8ef07b318