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 O= is made (which create host code)

That do not force the rebuild of config files, except with mm -B

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