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Extend linux/initrd command to load v3.9 above kernel initrd above 4G.

Need to use loadmemmax in grub.cfg to enable it, otherwise kernel/initrd will still be loaded under 4G.

sample grub.cfg

menuentry "testlinux 64bit entry serial" {
     loadmemmax 0x1ffffffff
     linux /linux ramdisk_size=262144 root=/dev/ram0 rw ip=dhcp console=uart8250,io,0x3f8,115200n8
     initrd /initrd.img
}

menuentry "testlinux 64bit entry serial efi" {
     loadmemmaxefi 0x1ffffffff
     linuxefi /linux ramdisk_size=262144 root=/dev/ram0 rw ip=dhcp console=uart8250,io,0x3f8,115200n8
     initrdefi /initrd.img
}