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 <[email protected]>

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
 
@@ -79,7 +79,7 @@ but without ACPI support) and `6.1`.
 After the command finishes, the kernels along with the corresponding KConfig
 used will be stored under `resources/$(uname -m)`.
 
-## 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
@@ -185,3 +185,44 @@ adjust the script(s) to suit your use case.
 
 You should now have a rootfs image (`ubuntu-22.04.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
+   ```
+
+   Firecracker support is available since FreeBSD 14.0 (released November 2023).
+
+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.