make_dtb.sh: Use $CPP instead of assuming that cpp is in $PATH

This fixes building in CheriBSD with a strict tmp path since we don't
bootstrap a cpp but pass the full path to clang-cpp instead.

While touching this file also fix all shellcheck warnings in make_dtb.sh.

Reviewed By: manu
Differential Revision: https://reviews.freebsd.org/D18376