tools/configure: Allow specifying python to be found from path

tools/configure: Allow specifying python to be found from path

./configure takes a PYTHON=... argument.  You can use this to specify
the python interpreter.  However, for no good reason, it expects an
absolute path.

Fix this.  The new logic is:
 * if not set, default to `python'
 * if not absolute, look it up with type -p
 * split into directory and executable name

The results in config/Tools.mk (which contains @PYTHON@ and
@PYTHONPATH@) are identical for both
  ./configure
  ./configure PYTHON=/usr/bin/python
so I assert this has no functional change except that now you can say
  ./configure PYTHON=python

In particular you can now say
  ./configure PYTHON=python2
  ./configure PYTHON=python3

The latter is useful if you want python3 (which should probably be the
default, but does not work right now).  The former is useful if you
want python2 but your distro has foolishly made "python" refer to
python3.

CC: Doug Goldstein <cardoe@cardoe.com>
CC: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wl@xen.org>