tools: moveconfig: make Slot.poll() more readable with helper methods

The Slot.poll() method is already complicated and a new feature
we are going to add will make it more difficult to understand
the execution flow.

Refactor it with helper methods, .handle_error(), .do_defconfig(),
.do_autoconf(), .do_savedefconfig, and .update_defconfig().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>