scriptutils: fix fetch_url() to use lowercase dummy recipe name

scriptutils: fix fetch_url() to use lowercase dummy recipe name

recipetool create (and hence devtool add) and devtool upgrade use
fetch_url() which creates a dummy recipe in order to fetch source.
Previously the random part of the name was using uppercase characters,
and this triggers a QA warning after OE-Core commit
4713f8b2c4f2c74239d284adcf1e59e61aa66576, so use lowercase instead as I
really should have in the first place.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>