stellaris: update bulk flash writes

stellaris: update bulk flash writes

Try to right-size the SRAM buffers, by not:
 - using them for very small writes
 - giving up when a large buffer isn't available
 - allocating buffers much larger than their data

Also don't:
 - bother loading the code unless we allocate the writebuffer too
 - be so verbose with messaging:
    * be more concise
    * reduce importance (e.g. DEBUG not WARNING)
    * remove duplication

The minimum buffer size is something of a guess.  It's eight
times smaller than before, almost the same size as the code
being downloaded.  It probably deserves some tuning.

Also, note an erratum affecting flash protection on some chips;
and narrow many over-wide lines affected by the above changes.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>