[efi] Prescroll the display after a failed wrapped ExitBootServices() call

[efi] Prescroll the display after a failed wrapped ExitBootServices() call

On some systems (observed with an HP Elitebook 840 G10), writing
console output that happens to cause the display to scroll will modify
the system memory map.  This causes builds with DEBUG=efi_wrap to
typically fail to boot, since the debug output from the wrapped
ExitBootServices() call itself is sufficient to change the memory map
and therefore cause ExitBootServices() to fail due to an invalid
memory map key.

Work around these UEFI firmware bugs by prescrolling the display after
a failed ExitBootServices() attempt, in order to minimise the chance
that further scrolling will happen during the subsequent attempt.

Signed-off-by: Michael Brown <mcb30@ipxe.org>