Factorize environment variables for config utilities

Instead of duplicating the definition of KCONFIG_AUTOCONFIG,
KCONFIG_AUTOHEADER and BUILDROOT_CONFIG, let's define them in a
COMMON_CONFIG_ENV variable, which is used by all the xconfig, gconfig,
menuconfig, nconfig, config, oldconfig, randconfig, allyesconfig,
allnoconfig, randpackageconfig, allyespackageconfig,
allnopackageconfig, defconfig, %_defconfig targets.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>