kbuild: create include/generated in silentoldconfig

The toplevel Makefile creates the directory if it runs silentoldconfig
automatically, but if run manually, it fails:

  $ make mrproper
  $ make defconfig && make silentoldconfig
  *** Default configuration is based on 'x86_64_defconfig'
  #
  # configuration written to .config
  #
  scripts/kconfig/conf -s arch/x86/Kconfig

  *** Error during update of the kernel configuration.
  ...

Move the mkdir command to the silentoldconfig target to make it work.

Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/kconfig/Makefile

@@ -27,6 +27,7 @@ oldconfig: $(obj)/conf
 	$< -o $(Kconfig)
 
 silentoldconfig: $(obj)/conf
+	$(Q)mkdir -p include/generated
 	$< -s $(Kconfig)
 
 localmodconfig: $(obj)/streamline_config.pl $(obj)/conf