It is needed for systems where python points to python3

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

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.

Absolute symbolic links refer to the host directory structure, while we
want them to refer to the external/ tree.  Since we have several trees,
we can't just rebase them, so walk down the tree trying to find a match.

Fragile and ugly, but seems to work.

Instead of specifying

  /foo/some/common/path/name: /bar/some/common/path/name

say

  /foo/&/some/common/path/name: /bar/&

The '/&/' tells mkbootfs where the common name begins; everything after that
is substituted for '&' in the host filesystem filename.

syntax:

   /internal/path/name/**: /host/path/name/**

Java likes upper case in class file names.