recipetool: create: support specifying a file as the local source

recipetool: create: support specifying a file as the local source

It is currently possible to specify a file (e.g. a tarball) on the local
disk as the source, but you have to know to put file:// in front of it.
There's really no need to force users to jump through that hoop if they
really want to do this so check if the specified source is a file and
prefix it with file:// if that's the case.

Also ensure the same works for "devtool add" at the same time.

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