x86/EFI: allow reboot= overrides when running under EFI

x86/EFI: allow reboot= overrides when running under EFI

By default we will always use EFI reboot mechanism when
running under EFI platforms. However some EFI platforms
are buggy and need to use the ACPI mechanism to
reboot (such as Lenovo ThinkCentre M57). As such
respect the 'reboot=' override and DMI overrides
for EFI platforms.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
- BOOT_INVALID is just zero
- also consider acpi_disabled in BOOT_INVALID resolution
- duplicate BOOT_INVALID resolution in machine_restart()
- don't fall back from BOOT_ACPI to BOOT_EFI (if it was overridden, it
  surely was for a reason)
- adjust doc change formatting

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>