sbp2: split sbp2_create_command_orb() for better readability

sbp2_create_command_orb() code cleanup:
 - add two helper functions to reduce nesting depth
 - omit the return value which was always ignored
 - remove unnecessary declaration from sb2.h

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>