add x86_64 target support

config/Config-images.in

include/kernel-defaults.mk

package/boot/grub2/Makefile

target/linux/x86_64/Makefile

target/linux/x86_64/base-files/etc/inittab

...t/linux/x86_64/base-files/lib/preinit/15_essential_fs_x86

target/linux/x86_64/base-files/lib/preinit/89_move_config

target/linux/x86_64/base-files/lib/upgrade/platform.sh

target/linux/x86_64/config-default

target/linux/x86_64/image/Makefile

target/linux/x86_64/image/gen_image_generic.sh

target/linux/x86_64/image/grub-early.cfg

target/linux/x86_64/image/grub-iso.cfg

target/linux/x86_64/image/grub.cfg

target/linux/x86_64/patches/011-tune_lzma_options.patch

target/linux/x86_64/patches/012-export_erase_write.patch