Revert "Move driver/linux to main repository."

Revert submission 8180-submodules

Reason for revert: Arm prefer to keep driver separate due to different license.
Reverted Changes:
I7b7ad6fc4:Move driver/linux to main repository.
I3e49fbd0b:Move driver/linux into the main repository rather ...

Change-Id: I355b14f80db8e06c4789b1d13cdb1e27a6ab0035
diff --git a/default.xml b/default.xml
index 62d8df0..074b09a 100644
--- a/default.xml
+++ b/default.xml
@@ -16,7 +16,6 @@
 
     <!-- TODO: Do we want these in the root or change paths? -->
     <linkfile src="build" dest="build" />
-    <linkfile src="driver" dest="driver" />
     <linkfile src="kokoro" dest="kokoro" />
     <linkfile src="inc" dest="inc" />
     <linkfile src="src" dest="src" />
@@ -30,6 +29,7 @@
   <project path="third_party/googletest" name="hafnium/third_party/googletest" clone-depth="1" />
   <project path="third_party/linux" name="hafnium/third_party/linux" clone-depth="1" />
 
+  <project path="driver/linux" name="hafnium/driver/linux" />
   <project path="project/reference" name="hafnium/project/reference" />
 
   <!-- Checkout this manifest as one of the projects. This will be used to init