mkisoimages.sh: Avoid creating temporary files in the current directory

[jrtc27]

Summary

Currently the current directory is the parent of the rootfs directory,
but this will change in order to support NO_ROOT builds that use a
metalog manifest, since those need to have the current directory be the
rootfs itself in order for the relative paths to be correct, and we do
not want the non-METALOG case (which passes the directory to makefs) to
pick up leftover temporary .img files from a previous failed build.

[brooks]

I wonder if these want a trap or exit handler

[jrtc27]

In D33998#768756, @brooks wrote:

I wonder if these want a trap or exit handler

The make-memstick.sh's all document that they deliberately don't clean up on error in order to aid in debugging what went wrong. Presumably mkisoimages.sh is the same, just without documenting it (though whether that was the original intent, who knows).

[emaste]

looks fine