twister: support custom test binaries

Have boards define what binaries should be kept for device testing. The
purpose of this is to reduce the amount of artifacts and just keep the
binaries needed for flashing a device, reducing the size of the final
payload.

A board can have twister keep multiple binaries needed for testing when
--prep-artifacts-for-testing option is used.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>