devtools: use a common prefix for temporary files

devtools: use a common prefix for temporary files

Some temporary files were generated in /tmp, others in the current
directory, and none was "dpdk prefixed".

All these files have a common path prefix now: $TMPDIR/dpdk.
TMPDIR is /tmp by default.

Note: the previous use of mktemp, with a template but without -t,
was generating a file in the current directory.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>