Migrate the build from the Google repo manifest to kas

Replace the repo + default.xml + setup-environment workflow with kas:

- add kas/forgefirm-glowforge.yml (Scarthgap layers pinned, meta-openglow
  as a local sibling) and kas/README.md
- remove default.xml, base/conf/forgefirm-bblayers.conf,
  scripts/setup-environment
- .gitignore the kas-managed layers/ and build caches
- BUILD.md: kas host setup and build instructions

BUILD.md

@@ -1,51 +1,76 @@
-# Build
-Instructions are for a Linux environment supported by Yocto.
-
-To get the build OpenGlow/ForgeFIRM you need to have `repo` installed:
-
-## Install the `repo` utility:
-
-```console
-mkdir ~/bin
-curl http://commondatastorage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
-chmod a+x ~/bin/repo
-PATH=${PATH}:~/bin
-```
-Add the following to the end of ```~/.bashrc``` to permanently add ```repo``` to your path:
-```console
-export PATH=~/bin:$PATH
-```
-## Download the BSP source:
-
-```console
-mkdir forgefirm
-cd forgefirm
-repo init -u https://github.com/ScottW514/forgefirm.git -b master -m default.xml
-repo sync
-```
-
-At the end of the commands you have all meta packages you need to build the basic firmware images.
-
-## Build the OpenGlow/ForgeFIRM image for factory control boards
-First time environment setup:
-```console
-(in forgefirm directory)
-MACHINE=glowforge DISTRO=forgefirm . setup-environment build
-bitbake forgefirm-image
-```
-Subsequent builds:
-```console
-(in forgefirm directory)
-. setup-environment build
-bitbake forgefirm-image
-```
-
-## Bootable Image:
-The bootable image can be found in ```forgefirm/build/tmp/deploy/images/```.
-
-It can be written directly to an SDCARD:
-```console
-(in forgefirm/build directory)
-cd tmp/deploy/images/glowforge
-sudo zcat forgefirm-image-glowforge.wic.gz | dd of=/dev/sdX bs=1M
-```
+# Build
+
+ForgeFIRM is built with [**kas**](https://kas.readthedocs.io/), which manages
+the Yocto layers and pins their versions. This replaces the old Google `repo`
++ `default.xml` workflow. The **forgefirm** repo is the base of the build.
+
+Builds run on a Linux host, or on **WSL2** (officially supported by Yocto —
+keep the tree on the native ext4 filesystem, not `/mnt/c`).
+
+## Host setup
+
+On Ubuntu/Debian (including WSL2), install the Yocto host packages and kas:
+
+```console
+sudo apt-get install -y gawk wget git diffstat unzip texinfo gcc build-essential \
+  chrpath socat cpio python3 python3-pip python3-pexpect xz-utils debianutils \
+  iputils-ping python3-git python3-jinja2 python3-subunit zstd liblz4-tool file \
+  locales libacl1 lz4 rsync
+sudo locale-gen en_US.UTF-8
+pipx install kas        # use pipx — Ubuntu 24.04 (PEP 668) blocks `pip install --user`
+```
+
+For other distros, see the
+[Yocto Project Quick Build](https://docs.yoctoproject.org/brief-yoctoprojectqs/index.html).
+Do not build as root (the Yocto sanity checks refuse it).
+
+## Get the sources
+
+Clone the repos as siblings (the kas config references `meta-openglow` at
+`../meta-openglow` while it is under active Scarthgap migration):
+
+```console
+git clone https://github.com/ScottW514/forgefirm.git
+git clone https://github.com/ScottW514/meta-openglow.git
+git clone https://github.com/ScottW514/kernel-module-glowforge.git
+```
+
+```
+openglow-forgefirm/
+├── forgefirm/                 ← base repo, build runs here
+├── meta-openglow/
+└── kernel-module-glowforge/
+```
+
+## Build the image
+
+```console
+cd forgefirm
+kas build kas/forgefirm-glowforge.yml
+```
+
+kas fetches the upstream layers into `forgefirm/layers/`, builds in
+`forgefirm/build/`, and produces the bootable image at:
+
+```
+forgefirm/build/tmp/deploy/images/glowforge/forgefirm-image-glowforge.wic.gz
+```
+
+For exact, reproducible layer versions, generate a lockfile once:
+
+```console
+kas lock kas/forgefirm-glowforge.yml
+```
+
+See [`kas/README.md`](kas/README.md) for details, the container-build option,
+and the Scarthgap migration backlog.
+
+## Write to an SD card
+
+```console
+cd build/tmp/deploy/images/glowforge
+sudo zcat forgefirm-image-glowforge.wic.gz | dd of=/dev/sdX bs=1M
+```
+
+To install onto the factory eMMC (dual-boot with the Glowforge firmware), see
+[`INSTALL.md`](INSTALL.md).