kconfig: fix oldconfig to generate .config.cmd

Since the recent update of Kconfig to the Kconfig infrastructure found
in 2.6.30 kernels, make oldconfig was broken because it didn't
generate the .config.cmd file. This is done by a call to the
conf_write_autoconf() function.

Moreover, this process was also broken because the
conf_get_autoconfig_name() function was reading the environment
variable BUILDROOT_AUTOCONF while the main Makefile is passing it as
KCONFIG_AUTOCONFIG. We also remove the fallback on
"$(BR2_DEPENDS_DIR)/config/auto.conf" since we are not able to expand
BR2_DEPENDS_DIR at this point. Therefore, the KCONFIG_AUTOCONFIG
envionment variable *must* be set.

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