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

.gitignore

@@ -0,0 +1,10 @@
+# kas-managed upstream layer checkouts (poky, meta-openembedded, meta-freescale...)
+/layers/
+
+# Yocto build output (incl. deploy/images) and caches
+/build/
+/downloads/
+/sstate-cache/
+
+# kas
+*.lock.yml.bak