efi_loader: update the timing of enabling and disabling EFI watchdog

UEFI specification requires that 5 minutes watchdog timer is
armed before the firmware's boot manager invokes an EFI boot option.
This watchdog timer is updated as follows, according to the
UEFI specification.

 1) The EFI Image may reset or disable the watchdog timer as needed.
 2) If control is returned to the firmware's boot manager,
    the watchdog timer must be disabled.
 3) On successful completion of EFI_BOOT_SERVICES.ExitBootServices()
    the watchdog timer is disabled.

1) is up to the EFI image, and 3) is already implemented in U-Boot.
This patch implements 2), the watchdog is disabled when control is
returned to U-Boot.

In addition, current implementation arms the EFI watchdog at only
the first "bootefi" invocation. The EFI watchdog must be armed
in every EFI boot option invocation.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>