Support magic number of dmg files

[sarensw]

dmg files have the magic number at the end of the file. MacPacker right now only supports magic numbers at a certain offset. As the size of the dmg is always different, a fixed offset cannot be set.

typedef struct {
  uint8_t  Signature[4];           // magic 'koly'
  ...
} __attribute__((packed, scalar_storage_order("big-endian"))) UDIFResourceFile;

Acceptance Criteria

• Allow magic numbers to be at the end of files

References

• https://en.wikipedia.org/wiki/Apple_Disk_Image