runqemu: report error if TMPDIR cannot be determined

runqemu: report error if TMPDIR cannot be determined

Invoking runqemu outside of the build directory for an
in-tree setup results in an empty TMPDIR because bitbake -e
cannot be run to find it.

A symptom of this problem is running runqemu and getting the
following error:

Error: Unable to find tunctl binary in <directory>

Catch this case and report the error to the user.

This fixes [YOCTO #1278]

Signed-off-by: Scott Garman <scott.a.garman@intel.com>