Disable loop unrolling in skein for sys/boot

When tsoome@ added skein support to the ZFS boot code and zfsloader, it
resulted in an explosion in code size, running close to a number of
limits.

The default for the C version of skein is to unroll all loops for
skein-256 and 512

Disabling the loop unrolling saves 20-28kb from each binary
boot1.efi
gptzfsboot
loader.efi
userboot.so
zfsloader

Reviewed by: emaste, tsoome
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D7826