mkimage : Split out and clean pbl_crc32 for use by other image types

The crc32 used by pblimgae is NOT the same as zlib crc32.

The pbl_crc32 is useful for other purposes in mkimage so split it out.

While we are about it, clean up redundant and confusing code.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>