Set a BOOTABLE variable to "-b" if MACHINE is not pc98, and use it for mkisoimages.sh instead of "-b" directly.
Set a BOOTABLE variable to "-b" if MACHINE is not pc98, and use it for
mkisoimages.sh instead of "-b" directly.

MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase