nitro_enclaves: Add Makefile for the Nitro Enclaves driver

Add Makefile for the Nitro Enclaves driver, considering the option set
in the kernel config.

Changelog

v9 -> v10

* Update commit message to include the changelog before the SoB tag(s).

v8 -> v9

* Remove -Wall flags, could use W=1 as an option for this.

v7 -> v8

* No changes.

v6 -> v7

* No changes.

v5 -> v6

* No changes.

v4 -> v5

* No changes.

v3 -> v4

* No changes.

v2 -> v3

* Remove the GPL additional wording as SPDX-License-Identifier is
  already in place.

v1 -> v2

* Update path to Makefile to match the drivers/virt/nitro_enclaves
  directory.

Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Link: https://lore.kernel.org/r/20200921121732.44291-16-andraprs@amazon.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>