gnu: raspberry-pi: Add a bootloader-chain for the Raspberry Pi and os examples.

gnu: raspberry-pi: Add a bootloader-chain for the Raspberry Pi and os examples.

* gnu/packages/raspberry-pi.scm (grub-efi-bootloader-chain-raspi-64): New
bootloader variable, capable to boot a Raspberry Pi over network or from a
local storage.
* gnu/system/examples/raspberry-pi-64.tmpl: New operating-system example.
* gnu/system/examples/raspberry-pi-64-nfs-root.tmpl: New operating-system
example for booting over network.
* Makefile.am (EXAMPLES): Register the new files.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>