A500 machine and kernel recipe

This PR introduces only the basic pieces of the A500 port of WebOS-Ports. It contains the following modifications:

 - the A500 machine configuration, stripped down a little bit because of the currently missing pieces 
 - the A500 kernel, which is a custom 3.1.10 android kernel
 - the chroot-script recipe, which is currently implemented to boot on the second partition of the external SDCard of the A500
 - a modification of kernel_android.bbclass, to be able to specify a custom CMDLINE in the fastboot image (I couldn't find a way to achieve that with EXTRA_MKBOOTIMG_ARGS)

With these pieces, the webos-port-dev-image bitbake target will produce a working A500 kernel fastboot image.