And Sougata,

One other thing:

I have already modified the approach slightly to make this "bootloader write
protect" software selectable. If you have access to the program, and give it an "unlock code", it puts itself in the "OK to run bootloader" mode. On the next power-up, the bootloader will run. If the bootloader isn't used during that power-up cycle OR the program is updated using the bootloader, the program then "locks" the bootloader again. In other words, you give the unlock code, then you have ONE chance to use the bootloader on the next power-up cycle.