UPSTREAM: 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.

Bug: 148277666
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
(cherry picked from commit 94cf8acc38e57caaba1329a255409b6d93936ba7)
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I414ffbfa92ee202f0c88757f896bf9c8811822d7