Rearrange vboot_reference directories to isolate external components.

This creates a new vboot_firmware subdirectory, and which contains the
entirety of the BIOS code. There shouldn't be anything in this directory
that is NOT required by the BIOS.

Review URL:
diff --git a/vboot_firmware/README b/vboot_firmware/README
new file mode 100644
index 0000000..2b0bff5
--- /dev/null
+++ b/vboot_firmware/README
@@ -0,0 +1,6 @@
+lib/ is stuff that the BIOS needs to link with.
+stub/ is stuff to be implemented by the BIOS.
+include/ describes the interfaces between the two parts.