altos: Validate boot chain start address

If the first block of boot memory has been smashed, and the start
address is bogus, don't bother trying to jump to the
application. This makes the system more resiliant to flash failures,
presuming the loader erases the first block, programs the other blocks
and then finally comes back to program the first block.

Signed-off-by: Keith Packard <keithp@keithp.com>