Add check for buffermode feature, and use it if present. Can be
turned off using -x no_blockmode.
* avr910.c: Implement buffermode test and usage.
* avrdude.1: Document -x no_blockmode.
* doc/avrdude.texi: (Ditto.)
git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk@771 81a1dc3b-b13d-400b-aceb-764788c761c2