Smaller crc for the boot loader.

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

Reviewed By: allanjude
Differential Revision: https://reviews.freebsd.org/D24225