Revert "rpi-firmware: updated to 98e5704."

This reverts commit 465d1730ba9f3d736a700e5c50a8caca2ff1fdae.
kernel get panic at alsa sound playback with this firmware.
rpi-firmware: reduce videocore memory to 16MB.

Add some comments about video/hdmi options to config.txt.
firewall: fix UPNPFW chain/rules creation order.
close core67.