sizeof(...) has type size_t which is long unsigned int on amd64, and so
the conversion specifier d without length modifier isn't suitable. There
is the length modifier 'z' for size_t arguments, but this is only
available on Linux. So casting to unsigned long and using 'lu' is the
portable fix.
This fixes:
rkboottool.c:215: warning: format ‘%d’ expects type ‘int’, but argument 4 has type ‘long unsigned int’
