compute_checksum
cksum = compute_checksum(extra + sizeof (bb_header_ext_t),
ext_hdr->checksum = compute_checksum((char *)dest, ext_hdr->size);
uint32_t compute_checksum(char *, uint32_t);
extern unsigned long compute_checksum(int *r_cksumerr, char *a_path);
mycksum = compute_checksum(&cksumerr, path);
unsigned long compute_checksum(int *r_err, char *path);