If the bootloader knows about a factory-reset or dev-mode button, use
them to allow stopping the boot process, and to signal the
factory-reset condition to infix.
The kernel's mounting of the rootfs may race against the underlying
device being probed - especially for media like MMC cards and USB
sticks.
Therefore, tell the kernel to wait for the root device to show up.
- Adapt boot scripts to exepect the split FIT header/SquashFS image
layout.
- Make sure that we can boot from any kind of block device (most
importantly virtio for GNS3).
- Rename net -> dhcp as we might have more kinds of netboots in the
future (http).
Ensure that whenever U-Boot is built:
- The chosen signing key and is converted to the expected format and
installed in the build tree.
- Infix's environment extensions are installed in the build tree
