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>