efi_loader: fix ExitBootServices

This patch lets the implementation of ExitBootServices conform to
the UEFI standard.

The timer events must be disabled before calling the notification
functions of the exit boot services events.

The boot services must be disabled in the system table.

The handles in the system table should be defined as efi_handle_t.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>