list-packageconfig-flags.py: add a script to list all PACKAGECONFIG flags

list-packageconfig-flags.py: add a script to list all PACKAGECONFIG flags

- This script will list available pkgs which have PACKAGECONFIG flags.
- If option '-f' is used, it will list available PACKAGECONFIG flags
  and all affected pkgs.
- If option '-a' is used, it will list all pkgs and PACKAGECONFIG
  information
- If option '-p' is used, it means list the pkgs with preferred version

EXAMPLE:
list-packageconfig-flags.py
PACKAGE NAME                               PACKAGECONFIG FLAGS
==============================================================
alsa-tools-1.0.26.1                        defaultval gtk+
avahi-ui-0.6.31                            defaultval python
bluez4-4.101                               alsa defaultval pie

list-packageconfig-flags.py -f
PACKAGECONFIG FLAG     PACKAGE NAMES
====================================
3g                     connman-1.16
avahi                  cups-1.6.3  pulseaudio-4.0
beecrypt               rpm-5.4.9  rpm-native-5.4.9

list-packageconfig-flags.py -a
==================================================
gtk+-2.24.18
/home/jiahongxu/yocto/poky/meta/recipes-gnome/gtk+/gtk+_2.24.18.bb
PACKAGECONFIG x11
PACKAGECONFIG[x11] --with-x=yes --with-gdktarget=x11,--with-x=no,${X11DEPENDS}

xf86-video-intel-2.21.9
/home/jiahongxu/yocto/poky/meta/recipes-graphics/xorg-driver/xf86-video-intel_2.21.9.bb
PACKAGECONFIG None
PACKAGECONFIG[xvmc] --enable-xvmc,--disable-xvmc,libxvmc
PACKAGECONFIG[sna] --enable-sna,--disable-sna

[YOCTO #4368]

(From OE-Core rev: 8d9e55e1fb073820c959f1797f3ad5a8932b441b)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>