efi_loader: Use sysreset instead of reset command

Use sysreset_walk_halt() directly from reset-after-capsule-on-disk
feature to reboot (cold reset) machine instead of using reset command
interface, since this is not a command.
Note that this will make CONFIG_EFI_CAPSULE_ON_DISK depending on
the CONFIG_SYSRESET.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>