Don't run the morse assertions forever, to save the battery.

- We run the morse code display for a limited number of times, then
  force a reset.

And some other changes:

- Disable the watchdog timer early in the startup sequence, with a function
  that is put in the ".init3" section by the linker.

- Flash a decimal point at startup, and also just before a forced reset.

- Display the reset reason from MCUSR in decimal point flashes.

- Fix an error where we called post() instead of postISR().

Signed-off-by: Russell Steicke <russells@adelie.cx>