freebsd: use python37

FreeBSD seems to use python37 by default now, which breaks the build
script.  Add python to the package list, to explicitly pick the version,
and also adapt the configure command line.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200106123746.18201-1-kraxel@redhat.com>