webos-image.bbclass: inherit core-image and remove boot_dd_append work around

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
webos.conf: Remove argp and x11 from default DISTRO_FEATURES

* argp is not used anymore
* x11 in pulseaudio pulls gtk+, libx11, libxfixses, ...

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>