depmod: pass -P $CONFIG_SYMBOL_PREFIX

On architectures which have symbol prefixes, depmod emits lots of
warnings like this:

WARNING: $module.ko needs unknown symbol $symbol

This is because depmod isn't being passed the -P <symbol_prefix>
arguments to specify the symbol prefix to ignore. This option is
included since the 3.13 release of module-init-tools.

Update scripts/depmod.sh to take extra arguments for the symbol prefix
(required but may be empty), and update the main Makefile to always pass
"$(CONFIG_SYMBOL_PREFIX)" to scripts/depmod.sh.

If the provided symbol prefix is non-empty, scripts/depmod.sh checks if
depmod --version reports module-init-tools with a version number < 3.13
otherwise it appends -P $SYMBOL_PREFIX to the depmod command line.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: linux-kbuild@vger.kernel.org
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: uclinux-dist-devel@blackfin.uclinux.org
Signe...