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>
Makefile: remove warnings on initial version check

Android.mk is using a SUBMAKE variable which is only
used to get busybox version as a CFLAG. This one has
the -s (--silent) makeflag. So ignore the KBUILD_OUTPUT
default value if this flag is present.

Change-Id: Ifcc11d148d6707a8c301b3d2ba0370b91215647e