Added lockbits to prevent flash corruption

In theory, the extended battery hack could cause flash memory
corruption at low voltages.  This is an extremely unlikely event, and
I have never seen it happen.

To eliminate the possibility of flash corruption, installation now
requires an extra step--"make install-lock".  This step sets lockbits
to prevent the xmas-icetube code from writing to flash memory.  See
the installation instructions in the README for details.