MFC r332345, r332346, r332661, r333005

MFC r332345, r332346, r332661, r333005

r332345:
 Fix makefs invocation after r331843.

r332346:
 Fix the position of $bootable so that -o platformid=efi applies correctly.

r332661:
 Generate hybrid ISO images for amd64.

 This keeps the existing El Torito entries for BIOS and UEFI boot code and
 adds a GPT in the ISO image's System Area containing boot code for BIOS that
 will load /boot/loader from the ISO filesystem and execute it. We then use
 etdump to find the EFI System Partition image in the El Torito catalog and
 add an entry to the GPT that allows EFI to find it.

r333005:
 Allow etdump, makefs and mkimg to be overridden.

 Recent changes to makefs and mkimg have led to situations where the
 disconnect between this script and the versions installed on the host cause
 failures. Provide a way to work around this that doesn't require the
 installation of new versions to the host system if that's not desired.

 With this change mkisoimages.sh will honour the $ETDUMP, $MAKEFS and $MKIMG
 environment variables but fall back to the previous behaviour of finding them
 within $PATH.

Sponsored by:	iXsystems, Inc.