[efi] Add "reboot" command for EFI

Abstract out the ability to reboot the system to a separate reboot()
function (with platform-specific implementations), add an EFI
implementation, and make the existing "reboot" command available under
EFI.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[cmdline] Add ability to perform a warm reboot

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