kconfig: make oldconfig is now less chatty

Previously when running "make oldconfig" we saw all the propmt lines
from kconfig and noone actully read this.

With this patch the user will only see output if there is new symbols.
This will be seen as "make oldconfig" runs which does not generate any output.

A typical run now looks like this:

$ make oldconfig
scripts/kconfig/conf -o arch/x86/Kconfig
$

If a new symbol is found then we restart the config process like this:
$ make oldconfig
scripts/kconfig/conf -o arch/x86/Kconfig
*
* Restart config...
*
*
* General setup
*
Prompt for development and/or incomplete code/drivers (EXPERIMENTAL) [Y/n/?] y
Local version - append to kernel release (LOCALVERSION) []
...

The bahaviour is similar to what we know when running the implicit
oldconfig target "make silentoldconfig".
"make silentoldconfig" are run as part of the kernel build process
if the configuration has changed.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>