Add FreeBSD kernel+rootfs build instructions

While I'm here, clarify that the existing instructions are for building
a Linux kernel and rootfs.

Signed-off-by: Colin Percival <cperciva@freebsd.org>

Author: cperciva

docs/rootfs-and-kernel-setup.md

@@ -1,6 +1,6 @@
 # Creating Custom rootfs and kernel Images
 
-## Creating a kernel Image
+## Creating a Linux kernel Image
 
 ### Manual compilation
 
@@ -72,7 +72,7 @@ config="resources/guest_configs/microvm-kernel-arm64-4.14.config"
 
 on an aarch64 machine.
 
-## Creating a rootfs Image
+## Creating a Linux rootfs Image
 
 A rootfs image is just a file system image, that hosts at least an init system.
 For instance, our getting started guide uses an ext4 filesystem image. Note
@@ -178,3 +178,45 @@ adjust the script(s) to suit your use case.
 
 You should now have a kernel image (`vmlinux`) and a rootfs image
 (`rootfs.ext4`), that you can boot with Firecracker.
+
+## Creating FreeBSD rootfs and kernel Images
+
+Here's a quick step-by-step guide to building a FreeBSD rootfs and kernel that
+Firecracker can boot:
+
+1. Boot a FreeBSD system.  In EC2, the
+   [FreeBSD 13 Marketplace image](https://aws.amazon.com/marketplace/pp/prodview-ukzmy5dzc6nbq)
+   is a good option; you can also use weekly snapshot AMIs published by the
+   FreeBSD project.  (Firecracker support is in FreeBSD 14 and later, so you'll
+   need FreeBSD 13 or later to build it.)
+
+   The build will require about 50 GB of disk space, so size the disk
+   appropriately.
+
+1. Log in to the FreeBSD system and become root.  If using EC2, you'll want to
+   ssh in as `ec2-user` with your chosen SSH key and then `su` to become root.
+
+1. Install git and check out the FreeBSD src tree:
+
+   ```sh
+   pkg install -y git
+   git clone https://git.freebsd.org/src.git /usr/src
+   ```
+
+   At present (July 2023) Firecracker support is only present in the `main`
+   branch.
+
+1. Build FreeBSD:
+
+   ```sh
+   make -C /usr/src buildworld buildkernel KERNCONF=FIRECRACKER
+   make -C /usr/src/release firecracker DESTDIR=`pwd`
+   ```
+
+You should now have a rootfs `freebsd-rootfs.bin` and a kernel `freebsd-kern.bin`
+in the current directory (or elsewhere if you change the `DESTDIR` value) that
+you can boot with Firecracker.  Note that the FreeBSD rootfs generated in this
+manner is somewhat minimized compared to "stock" FreeBSD; it omits utilities
+which are only relevant on physical systems (e.g., utilities related to floppy
+disks, USB devices, and some network interfaces) and also debug files and the
+system compiler.