chromium: Update README and dependencies for kirkstone (#800)

See the discussion in
#789.

Author: MaxIhlenfeldt

meta-chromium/README.md

@@ -72,21 +72,20 @@ host. clang-native from the meta-clang layer is used to build those binaries.
 Additionally, make sure the machine being used to build Chromium is powerful
 enough: a x86-64 machine with at least 16GB RAM is recommended.
 
-### dunfell-specific requirements
+### kirkstone-specific requirements
 
-The dunfell OE/Yocto branch is an LTS release, which is often at odds with
+The kirkstone OE/Yocto branch is an LTS release, which is often at odds with
 Chromium's release model because it often requires recent versions of certain
 recipes to build correctly.
 
-This is particularly a problem for the toolchain (i.e. LLVM) and, at times,
-node.js. As such, dunfell users **must** ensure the following:
-* Chromium needs clang >= 12 and for that, meta-clang's **dunfell-clang12**
-  branch needs to be used.
-* Since Chromium 112, at least Nodejs 14 is required for parts of the build.
-  meta-oe needs to be at least a commit
-  116bfe8d5e5851e7fc5424f40da8691a19c5b5ee ("nodejs: make 14.18.1 available but
-  not default") and `PREFERRED_VERSION_nodejs-native = "14.%"` needs to be
-  added to `conf/local.conf`.
+This is particularly a problem for the toolchain (i.e. LLVM/clang and Rust).
+Because meta-clang's kirkstone branch currently only provides clang 14, we are
+unable to support Chromium versions higher than 120 due to those versions
+requiring a more recent clang version. Also, kirkstone users **must** ensure the
+following:
+* Chromium needs a somewhat recent version of Rust, and for that
+  meta-lts-mixins' **kirkstone/rust-1.68** or **kirkstone/rust-1.70** branch
+  needs to be used.
 
 ## PACKAGECONFIG knobs
 

meta-chromium/conf/layer.conf

@@ -11,4 +11,4 @@ BBFILE_PRIORITY_chromium-browser-layer = "7"
 LAYERVERSION_chromium-browser-layer = "1"
 LAYERSERIES_COMPAT_chromium-browser-layer = "kirkstone langdale mickledore nanbield scarthgap"
 
-LAYERDEPENDS_chromium-browser-layer = "clang-layer core openembedded-layer"
+LAYERDEPENDS_chromium-browser-layer = "clang-layer core openembedded-layer lts-rust-mixin"