runqemu: improve auto-detection of rootfs filenames

runqemu: improve auto-detection of rootfs filenames

This refactors the way rootfs filenames are auto-detected when you
run the runqemu script without an explicit rootfs filename argument.
It allows the script to use rootfs files generated by hob, and when
there are mutliple rootfs files to choose from, it will pick the
most recently created one.

Fixes [YOCTO #1437].

(From OE-Core rev: 094453f443ed592d814dfb4182a0a02f0a2552e4)

Signed-off-by: Scott Garman <scott.a.garman@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>