MGS: Support SP updates that include an aux flash image

[jgallagher]

This implements the MGS side of delivering SP updates that may contain a separate auxflash image:

1. We extract the auxflash tlvc file from the hubris archive. If it doesn't exist, skip to step 5.
2. We extract the CHCK value via tlvc
3. We send the CHCK value and the sizes of the auxflash and sp images to the SP
4. We wait until the SP responds with the result of its scan for a matching CHCK
   a. If a CHCK match was found, go to step 5.
   b. If a CHCK match was not found, we send the aux flash image in chunks.
5. Send the SP image in chunks.

I moved all of this logic into a new update submodule under single_sp to try to keep it a little organized.

This builds on #1761 and should be merged after it.

[andrewjstone]

Very clean!