Increase the amd64 ISO ESP file size from 800KB to 1024KB.

Increase the amd64 ISO ESP file size from 800KB to 1024KB.

At some poing over the last week, the bootx64.efi file has grown
past the 800KB threshold, resulting in being unable to copy it to
the EFI/BOOT directory.

 # stat -f %z efiboot.znWo7m
 819200
 # stat -f %z stand-test.PIEugN/EFI/BOOT/bootx64.efi
 842752

The comment in the script that creates the ISOs suggests that 800KB
is the maximum allowed for the boot code, however I was able to
boot an ISO with a 1024KB boot partition.  Additionally, I verified
against an ISO from OtherOS, where the boot EFI partition is 2.4MB.

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