kbuild: move modpost out of 'scripts' target

I am eagar to build under the scripts/ directory only with $(HOSTCC),
but scripts/mod/ highly depends on the $(CC) and target arch headers.
That it why the 'scripts' target must depend on 'asm-generic',
'gcc-plugins', and $(autoksyms_h).

Move it to the 'prepare0' stage. I know this is a cheesy workaround,
but better than the current situation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile

@@ -37,9 +37,8 @@ build_unifdef: $(obj)/unifdef
 	@:
 
 subdir-$(CONFIG_MODVERSIONS) += genksyms
-subdir-y                     += mod
 subdir-$(CONFIG_SECURITY_SELINUX) += selinux
 subdir-$(CONFIG_GDB_SCRIPTS) += gdb
 
 # Let clean descend into subdirs
-subdir-	+= basic dtc kconfig package gcc-plugins
+subdir-	+= basic dtc kconfig mod package gcc-plugins