grub_efi_finish_boot_services not spec-compliant

[Seth Goldberg]

Hi there,

  On some systems, ExitBootServices() will fail due to a change in the memory 
map during execution of the boot services call.   The UEFI spec says that in 
that event, GetMemoryMap() should be repeated, then the new key passed to 
another call to ExitBootServices().  Note that the spec doesn't give an upper 
bound on the number of times to do this dance.  The logic in 
grub_efi_finish_boot_services() just tries once, leading to a failure to 
properly shut down boot services and various issue booting UEFI-compliant 
operating systems.

  --S