fix bug preventing reading of last byte of sectors.
Revert "make vertical scaling more flexible, similar to recent work on"

This reverts commit 29d96161f3b00b27f5e1070b23cc362a10f07263.
stop 16-bit branches eating one too many instruction bytes.
fix destination address of 16 bit branches.
fix BBS/BBR instructions.
lengthen SD card reset time to hopefully take account of higher
clock speed.