Add cheriot-rtos for sencha platform.

Change-Id: Id040387047aa064141d50ad58760cc5fae94583c

manifest.xml

diff --git a/manifest.xml b/manifest.xml
index 201ffdd..b398627 100644
--- a/manifest.xml
+++ b/manifest.xml
@@ -84,6 +84,9 @@
   <project name="hw/matcha"                       path="hw/matcha" />
   <project name="3p/ip/isp"                       path="hw/ip/isp" />
 
+  <!-- Sencha -->
+  <project name="3p/cheriot-rtos"                 path="sw/cheriot-rtos" />
+
   <!-- TockOS -->
   <project name="3p/tock/tock"       dest-branch="master" path="sw/tock" />
   <project name="3p/tock/libtock-rs" dest-branch="master" path="sw/libtock-rs" />