kdb: Remove unused "argcount" param from kdb_bt1(); make btaprompt bool

The kdb_bt1() had a mysterious "argcount" parameter passed in (always
the number 5, by the way) and never used.  Presumably this is just old
cruft.  Remove it.  While at it, upgrade the btaprompt parameter to a
full fledged bool instead of an int.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>