Proposed change of kdb key on serial console

[Keith Owens]

Currently you invoke kdb on a serial console by ctrl-A.  This causes
problems for people using emacs bindings in bash, ctrl-A is start of
line.  It is very annoying when typing on the serial console to
suddenly drop into kdb without intending to.

In the next major release of kdb, the serial console sequence will
change to <escape>KDB, in upper case.  You can revert to ctrl-A by
changing kdb_serial_str in drivers/char/serial.c.  Complain now if this
will cause any major problems.