LED Blink on r2 board is unreliable

[roscopeco]

The interrupt-driven LED blink on the r2 board with firmware 2.0 is unreliable, and will sometimes stop blinking (especially noticeable when the computer is under load, such as when receiving via Kermit).

This appears to be a simple issue in the firmware. Specifically, the TICK_HANDLER in bootstrap.asm is writing the byte part of a register, but testing a word. So this code:

move.b  SDB_INTFLAGS,D1
tst     D1                        ; Is INTFLAGS zero?

should actually use an explicit size for the tst instruction (which otherwise defaults to test.w):

move.b  SDB_INTFLAGS,D1
tst.b   D1                        ; Is INTFLAGS zero?

I will push a branch to fix this issue shortly.