Update git submodules

* Update openstack-virtual-baremetal from branch 'master'
  - Merge "Don't use diskimage-builder to build IPXE image"
  - Don't use diskimage-builder to build IPXE image
    
    IPXE's build process already creates bootable images, so we just use one of
    those. This builds much faster, has far fewer dependencies, and produces
    a much smaller image. The largest difference in output is that the
    resulting image no longer uses grub, but as we weren't using any
    features of grub this also means a faster boot time.
    
    For reference, a full build takes 62 seconds on my laptop. The new
    Makefile also handles dependencies much better so, for example, an
    incremental rebuild after changing the embedded script takes under 3
    seconds.
    
    The ipxe repo changes to being a submodule, which continues to allow us
    to pin it to a specific commit.
    
    We no longer build the -41 image, as that was oddly specific to some
    undescribed use-case. It's also trivial for whoever needed it to do
    themselves if the use case is still relevant.
    
    Change-Id: I5a6a990e6966d6eeaa2aa87cd89bfbac6738f6ee

openstack-virtual-baremetal

@@ -1 +1 @@
-Subproject commit d91520efc3118a9a16ed04d9b5f87be5d6388ee6
+Subproject commit 6f2f0a6377c12eff0b48fc7b458937ee52f1184e