mkbootfs.py supports wildcard entries, looking like:
/jetty/**: ../../../tmp/jetty/**

The name of the generated files in the bootfs looked like this:
                name + relpath + '/' + filename

where "name" is "/jetty/", relpath is the directory under it
(so for /jetty/foo/bar/yo, relpath is "foo/bar") and filename
is the file name (in this example "yo"). The problem is that
when relpath=="", i.e., files directly under /jetty, an extra
slash was generated - e.g., /jetty//something. When this extra
slash was written to the filesystem it confused readdir(),
causing it to return twice for each file (once with an empty
filename, and a second time with the real filename).

This patch avoid the extra slash when relpath is empty.