added grub.cfg for grubx64.efi

Configfile redirects to our grub.cfg.

boot/grub/x86_64-efi/grub.cfg

+insmod part_acorn
+insmod part_amiga
+insmod part_apple
+insmod part_bsd
+insmod part_dfly
+insmod part_dvh
+insmod part_gpt
+insmod part_msdos
+insmod part_plan
+insmod part_sun
+insmod part_sunpc
+configfile /grub/grub.cfg