util/cbfstool: Use `%zu` instead of `%ld` for size_t arguments

cbfstool fails to built under 32-bit platforms since commit

    aa2f739a cbfs: fix issues with word size and endianness.

due to the use of '%ld' format specifier on size_t, which on these
platforms is only 32-bit.

No error is seen though, when cbfstool is built, when building a coreboot
image, where it is put in `build/cbfstool`.

Use the length modifier `z` for size_t arguments, and cast to size_t where
appropriate.

Change-Id: Id84a20fbf237376a31f7e4816bd139463800c977
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/5388
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@gmail.com>