kbuild: make missing $DEPMOD a Warning instead of an Error

When $DEPMOD is not found, only print a warning instead of exiting
with an error message and error status:

Warning: 'make modules_install' requires /sbin/depmod. Please install it.
This is probably in the kmod package.

Change the Error to a Warning because "not all build hosts for cross
compiling Linux are Linux systems and are able to provide a working
port of depmod, especially at the file patch /sbin/depmod."

I.e., "make modules_install" may be used to copy/install the
loadable modules files to a target directory on a build system and
then transferred to an embedded device where /sbin/depmod is run
instead of it being run on the build system.

Fixes: 934193a6 ("kbuild: verify that $DEPMOD is installed")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: H. Nikolaus Schaller <hns@goldelico.com>
Cc: stable@vger.kernel.org
Cc: Lucas De Marchi <lucas.demarchi@profusion.mobi>
Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Chih-Wei Huang <cwhuang@linux.org.tw>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>