kconfig: make defconfig is no longer chatty

make defconfig generated a lot of output
then noone actually read.
Use conf_set_all_new_symbols() to generate the default
configuration and avoid the chatty output.

A typical run now looks like this:
$ make  defconfig
*** Default configuration is based on 'i386_defconfig'
arch/x86/configs/i386_defconfig:13:warning: trying to assign nonexistent symbol SEMAPHORE_SLEEPERS
arch/x86/configs/i386_defconfig:176:warning: trying to assign nonexistent symbol PREEMPT_BKL
...
arch/x86/configs/i386_defconfig:1386:warning: trying to assign nonexistent symbol INSTRUMENTATION
$

As an added benefit we now clearly see the warnings generated
in the start of the process.

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