Populating initramfs with BusyBox and uClibc.

This tutorial focuses on using initramfs.  It covers why you'd want
to, how to go about it, and what tools are available, with a special emphasis
on using BusyBox and uClibc to easily create initramfs images.

The tutorial starts by covering differences between initramfs and initrd
(ramfs vs block, archive vs image, bundled vs unbundled, init vs linuxrc),
and some advantages of the way initramfs moves more control to userspace (with
examples for laptops, embedded systems, clusters, and mainframes).

Next comes creating a viable initramfs image, which involves knowledge
of the Linux kernel (building, packaging, and calling conventions), populating
root filesystems (enumerating and specifying shared libraries, filesystem
hierarchy, and the special considerations for PID 1 and /dev/console), and
new tools (such as mdev and switch_root) specifically useful in an initramfs
environment.

After demonstrating a "hello world" initramfs image, the
tutorial shows how to debug it under both User Mode Linux and QEMU.  It
covers convincing gcc to build against something other than glibc (via wrapper
script or creating a new toolchain), and why developers might want to do this.
It demonstrates simple device and network probing from initramfs (with
mdev and udhcpc), handing off control to another init program in another
root partition (pivot_root), and extracting an initramfs from an existing
kernel image for analysis.