transparent-gateway: Clarify building docs on supported board

For now the gateway blueprint testing is limited to raspberrypi4-64. We
are not actively excluding other boards, but at least wifi and a USB
host for an OpenThread dongle is needed. Better state the tested board
only in the docs until we expand the testing.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

docs/blueprints/transparent-gateway.rst

@@ -68,4 +68,4 @@ Get sources
 
    .. code-block:: bash
 
-      user@pc:~/ohos$ DISTRO="allscenarios-linux-blueprint-gateway" bitbake blueprint-gateway-image
+      user@pc:~/ohos$ DISTRO="allscenarios-linux-blueprint-gateway" MACHINE=raspberrypi4-64 bitbake blueprint-gateway-image