[oe] support another hardware

[Sledz, Steffen]

> a) find out which kernel version your custom kernel is (e.g 2.6.27)
> b) extract the diff (diff -Nurd linux-2.6.27/ linux-2.6.27-mymachine/)
> c) add SRC_URI_append_mymachine = "file://mymachine.diff" to 
>    linux_2.6.27.bb
> d) put defconfig in packages/linux/linux-2.6.27/mymachine/defconfig

It seems to be necessary to add DEFAULT_PREFERENCE_mymachine = "1" too. Right?