Leez: capture certified firmware

- Freeze build scripts revision
- Update Makefile link
- Add a few words in the readme
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

Leez-P710.xml

@@ -8,7 +8,7 @@
 
 	<project path="atf" remote="tfa" name="trusted-firmware-a.git" revision="refs/tags/v2.4"/>
 	<project path="u-boot" remote="u-boot" name="u-boot.git" revision="refs/tags/v2021.07"/>
-	<project path="build" remote="systemready" name="firmware-build/rk3399-build">
-		<linkfile src="Leez-P710.mk" dest="build/Makefile" />
+	<project path="build" remote="systemready" name="firmware-build/rk3399-build" revision="refs/tags/leez-21.08">
+		<linkfile src="leez.mk" dest="build/Makefile" />
 	</project>
 </manifest>

README.md

@@ -24,6 +24,12 @@ $ repo init -u https://gitlab.arm.com/systemready/firmware-build/rk3399-manifest
 $ repo sync -j4 --no-clone-bundle
 ```
 
+To obtain the same firmware as the one used for Leez certification, use this `repo init` command:
+
+```
+$ repo init -u https://gitlab.arm.com/systemready/firmware-build/rk3399-manifest -m Leez-P710.xml -b refs/tags/leez-21.08
+```
+
 ## 3. Get the Aarch64 toolchain (optional)
 ```
 $ cd <New_Dir>/build/