Save 7k of text space by using simpler crc32 for standalone case.

[imp]

Summary

We don't need all that fancy optimization in the boot loader, so use a
simplified version of the CRC function. We could save more by doing it one bit
at a time rather than 32, but this is the biggest savings at the smallest
performance hit.

With LUA and verfied exec, gptboot, gptzfsboot and friends are pushing the ~530k
limit and every little bit helps.

[allanjude]

This seems like a good way to cut down the size

Reviewed By: allanjude