rockchip.inc: set KERNEL_IMAGETYPE="zImage"

The variable KERNEL_IMAGETYPE is required by rockchip-update-img.bbclass but
its default value is not correctly loaded by the build system, so we have to
add it into the machine configuration file.
recipies-devtools: Add net-kexec package

This installs a script which is able to kexec a kernel through the network from
user space. As the proprietary bootloader is limited and does not offer such a
feature, this is a good alternative for developers.
recipies-core: Add rockchip-basic-image distro

This adds a basic and minimalistic distro based on core-image-minimal.
It adds package management and ssh-server features and contains development
tools.