twister: add a --flash-command flag

Currently twister requires west to flash a board, either directly using
west-flash or indirectly through the cmake flash target.

Add an option to specify a custom flash script, this is passed a
build-dir and board-id flags so it can be used to implement custom
flashing scripts in a system that does not use west.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>