kbuild: fix silentoldconfig recursion

kconfig-fix-config-dependencies causes this:

make CC=cc  KBUILD_VERBOSE=1 -C /usr/src/25 SUBDIRS=/home/akpm/NVIDIA-Linux-x86_64-1.0-8762-pkg2/usr/src/nv modules
make -f /usr/src/devel/Makefile silentoldconfig
make -f /usr/src/devel/Makefile silentoldconfig
make -f /usr/src/devel/Makefile silentoldconfig

The basic problem is if we compile external modules, config-targets isn't
set which can cause recursive calls to silentoldconfig to update the
kernel configuration.

Bail out and ask the user to update manually.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>