efi_loader: use TPL_NOTIFY for network timer event

We use a timer to poll the network.

iPXE is used for booting from iSCSI drives. It has been changed to run at
TPL_CALLBACK most of the time (which is not what the UEFI spec
recommends).

By changing our timer to TPL_NOTIFY we can ensure that it is nevertheless
executed.

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