st3m_tar: only update sys files if changed
Becuase the tar extractor is streaming, this accomplishes that by:
- doing a big malloc() of the size of the file (which, given the files we have, and how early we are in the boot process, is perfectly reasonable)
- putting the file contents in there
- opening the file on disk and comparing to see if updating is needed
- writing only in that case
In hindsight the part of reading the file from disk could also be a big malloc but I went for 4kbyte buffers.
The performance difference between writing and doing anything else is so big that I didn't really need to care about making this code fast.