kbuild: modules_install: warn when missing System.map file

If there is no System.map file for "make modules_install",
scripts/depmod.sh will silently exit with success, having done
nothing.  Since this is an unexpected situation, change it to
report a Warning for the missing file.  The behavior is not
changed except for the Warning message.

The (previous) silent success and new Warning can be reproduced
by:
$ make mrproper; make defconfig
$ make modules; make modules_install

and since System.map is produced by "make vmlinux", the steps
above omit producing the System.map file.

Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>