Refactor multiboot related stuff

[binarymaster]

• Recently added ReactOS boot support partly implements multiboot support, which needs to be refactored and generalized to allow booting any multiboot-compliant binaries which Cromwell can read from the accessible storage devices
• Refactoring should be done with care to ReactOS boot (it should not be broken, or become more complicated)
• Some multiboot structures are duplicated in LoadReactOS.c and mb_header.h / mb_info.h header files, this needs to be fixed