Fixing the kernel target for raspberrypi (zImage)

Removing uboot things about it too.
raspberrypi platform: adding a missing uboot attr

It's required, even if set to null, along all nixpkgs.
Update the raspberry pi kernel to one newer.
git: adding an option not to build the manual

asciidoc+xsltproc take hours to run
Raspberrypi platform: make ext4 in kernel

I also make other modules in-kernel. I originally copied this
from the sheevaplug kernel, but the sheevaplug runs the initrd
and the raspberrypi not.