gst1-plugins-bad: fix openh264 handling

Commit 301e8ffb (gst1-plugins-bad: update configure options) added a sub
option for openh264, but didn't propagate the toolchain dependencies or add
any configure options.

Fix this and while we're at it also add a help text for the option.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>