arm-bsp,kas: corstone1000: enable External System based on new yml file

Create new yml file "corstone1000-extsys.yml" which adds "corstone1000-extsys" as
new MACHINE_FEATURE.
Based on this, external system components can be enabled or disabled from the
Linux Kernel and U-Boot.

Reason for change:
DT-schema test is failing for the SystemReady-IR v2.0 certification because
device tree binding for remoteproc dts node corresponds to external system has
not been upstreamed in the Linux Kernel yet.
So, it has been decided to make enablement of external system configurable in
order to make Corstone1000 FVP SystemReady-IR v2.0 certifiable.

Signed-off-by: Harsimran Singh Tungal <harsimransingh.tungal@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/recipes-kernel/linux/linux-arm-platforms.inc

@@ -35,13 +35,21 @@ SRC_URI:append:corstone1000 = " ${@bb.utils.contains('MACHINE_FEATURES', \
                                                             'file://corstone1000_kernel_debug.cfg', \
                                                             '', \
                                                              d)}"
-
 SRC_URI:append:corstone1000 = " \
-    file://extsys.cfg \
-    file://0001-remoteproc-Add-Arm-remoteproc-driver.patch \
-    file://0002-arm64-dts-Add-corstone1000-external-system-device-no.patch \
-    file://0003-dt-bindings-remoteproc-Add-Arm-remoteproc.patch \
-    "
+    ${@bb.utils.contains( \
+              'MACHINE_FEATURES', \
+              'corstone1000-extsys', \
+              ' \
+		  file://extsys.cfg \
+		  file://0001-remoteproc-Add-Arm-remoteproc-driver.patch \
+		  file://0002-arm64-dts-Add-corstone1000-external-system-device-no.patch \
+		  file://0003-dt-bindings-remoteproc-Add-Arm-remoteproc.patch \
+              ', \
+              '', \
+              d \
+        ) \
+    } \
+"
 
 # Default kernel features not needed for corstone1000
 # otherwise the extra kernel modules will increase the rootfs size