powerpc: fix release image building for Apple partitions

awk changed somewhere between 14 and 15 and it stopped accepting
a hexadecimal number as its input - it will always return 0.
This results in a very badly written apple boot block.

So just remove it; do the math in shell.

PR: kern/292341
Differential Revision: https://reviews.freebsd.org/D54639
Reviewed by: imp
MFC after: 1 week