kbuild: source include/config/auto.conf instead of ${KCONFIG_CONFIG}

As commit 423a8155 ("kbuild: Fix reading of .config in
link-vmlinux.sh") addressed, some shells fail to perform '.' if
${KCONFIG_CONFIG} does not contain a slash at all.

Instead, we can source include/config/auto.conf, which obviously
contain slashes, and we do not expect its file path overridden by
a user. Perhaps, the performance might be slightly better since
unset CONFIG options are stripped from include/config/auto.conf.

scripts/setlocalversion already works this way.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/adjust_autoksyms.sh

@@ -39,14 +39,7 @@ case "$KBUILD_VERBOSE" in
 esac
 
 # We need access to CONFIG_ symbols
-case "${KCONFIG_CONFIG}" in
-*/*)
-	. "${KCONFIG_CONFIG}"
-	;;
-*)
-	# Force using a file from the current directory
-	. "./${KCONFIG_CONFIG}"
-esac
+. include/config/auto.conf
 
 # Generate a new ksym list file with symbols needed by the current
 # set of modules.

scripts/link-vmlinux.sh

@@ -171,14 +171,7 @@ if [ "$1" = "clean" ]; then
 fi
 
 # We need access to CONFIG_ symbols
-case "${KCONFIG_CONFIG}" in
-*/*)
-	. "${KCONFIG_CONFIG}"
-	;;
-*)
-	# Force using a file from the current directory
-	. "./${KCONFIG_CONFIG}"
-esac
+. include/config/auto.conf
 
 # Update version
 info GEN .version