Move Linux build rule to its own folder, update to v5.4.2

The Linux build rule was previously in //third_party/ which meant it was
built in the same folder as other third-party projects. Move its build
rule to its folder to avoid clashes.

Also update Linux to v5.4.2 and update submodule refs.

Change-Id: I32598092c3ef400677ccdb22d0b92f68de113d39
diff --git a/test/linux/BUILD.gn b/test/linux/BUILD.gn
index a94af7f..681830e 100644
--- a/test/linux/BUILD.gn
+++ b/test/linux/BUILD.gn
@@ -59,7 +59,7 @@
 
   manifest = "manifest.dts"
   primary_name = "vmlinuz"
-  primary_vm = "//third_party:linux__prebuilt"
+  primary_vm = "//third_party/linux:linux__prebuilt"
   primary_initrd = ":linux_test_initrd"
   secondary_vms = [ [
         "socket0",