infra/distro-boot: update distro installation instructions

Add instructions on setting up of the 'MODEL' environment variable to
specify the absolute path of the platform FVP.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I56adb6316fd2f4afc5cab2e3176f79cf527738a2

docs/infra/common/distro-boot.rst

@@ -102,6 +102,12 @@ following locations (select an image built for aarch64 architecture):
 
 The commands used to begin the distro installation are:
 
+- Set ``MODEL`` path before launching the model:
+
+  ::
+
+    export MODEL=<absolute path to the platform FVP binary>
+
 - If platform is SGI-575:
 
   ::