kconfig: get target architecture from config file

This patch makes working with kernel cross-compilation little bit easier.
After initial configuration no special environment variables are required,
because both ARCH and CROSS_COMPILE are stored in .config file:

# make ARCH=arm CROSS_COMPILE=arm-none-eabi- defconfig
# make

This especially useful together with option O=dir which allows to create
separate directory for each target architecture and kernel flavor:

# make O=out/arm ARCH=arm CROSS_COMPILE=arm-none-eabi- defconfig
# cd out/arm
# make menuconfig
# make

To avoid recursion root Makefile reads ARCH directly from .config using
scripts/config because include/config/auto.conf is not yet included here.

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>