disk: efi: ignore 'IGNOREME' GPT header found on cros eMMCs

Some ChromeOS devices (atleast veyron speedy) have the first 8MiB of
the eMMC write protected and equipped with a dummy 'IGNOREME' GPT
header - instead of spewing error messages about it, just silently
try the backup GPT.

Note: this does not touch the gpt cmd writing/verifying functions,
those will still complain.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>