fdt: Make sure there is no stale initrd left

fdt: Make sure there is no stale initrd left

Although if we don't setup an initrd, there could be a stale initrd
setting from the previous boot firmware in the live device tree. So,
make sure there is no setting left if we don't want an initrd.

This can happen when booting on a Raspberry Pi. The boot firmware can
happily load an initrd before us and configuring the addresses in the
live device tree we get handed over.

Especially the setting `auto_initramfs` in config.txt is dangerous.
When enabled (default), the firmware tries to be smart and looks for
initramfs files.

Signed-off-by: Richard Weinberger <richard@nod.at>