fix bug with m and M displaying one row too many.
fix blink attribute so that it frame locked to every 60 frames.
This gives 1Hz, and no tearing of display caused by blink occurring
mid-frame that occurred with the old cycle counter based method.
Fix missed bytes when reading sdcard while CPU accesses fast RAM.
Side effect is that sector buffer cannot be read or written to
while busy flag is set. Issue #24