[coreboot] Add mkelfimage and support for generating kernel+initrd ELF payloads to OpenEmbedded

[Raymond Danks]

So, It's been quite a while (>5 years) since I have submitted anything 
substantial to the OE project.  It looks like I may be getting back into 
this some, however.  I have a couple of minor changes/additions after 
this, so please let me know if I am missing the process somehow.

I am CC-ing the coreboot list on this particular change because of the 
implication for the coreboot project.