build-buildroot: add the --no-overlay option

Initial motivation: modprobe. More to come most likely.

README.adoc

@@ -3172,9 +3172,7 @@ We are not installing out custom `./build-modules` modules there, because:
 
 * we don't know the right way. Why is there no `install` or `install_modules` target for kernel modules?
 +
-This can of course be solved by running Buildroot in verbose mode, and copying whatever it is doing.
-+
-See also: https://askubuntu.com/questions/299676/how-to-install-3rd-party-module-so-that-it-is-loaded-on-boot
+This can of course be solved by running Buildroot in verbose mode, and copying whatever it is doing, initial exploration at: https://stackoverflow.com/questions/22783793/how-to-install-kernel-modules-from-source-code-error-while-make-process/53169078#53169078
 * we would have to think how to not have to include the kernel modules twice in the root filesystem, but still have <<9p>> working for fast development as described at: <<your-first-kernel-module-hack>>
 
 === kmod
@@ -11322,17 +11320,15 @@ A custom build script can give you more flexibility: e.g. the package can be mad
 
 Source: link:buildroot_packages/kernel_modules/[]
 
-An example of how to kernel modules in Buildroot.
-
-Procedure described in detail at: https://stackoverflow.com/questions/40307328/how-to-add-a-linux-kernel-driver-module-as-a-buildroot-package/43874273#43874273
+An example of how to use kernel modules in Buildroot.
 
 Usage:
 
 ....
-rm -rf "$(./getvar out_rootfs_overlay_dir)/lib/modules"
 ./build-buildroot \
   --build-linux \
   --config 'BR2_PACKAGE_KERNEL_MODULES=y' \
+  --no-overlay \
   -- \
   kernel_modules-reconfigure \
 ;
@@ -11346,10 +11342,14 @@ Then test one of the modules with:
 
 Source: link:buildroot_packages/kernel_modules/buildroot_hello.c[]
 
-The `rm -rf` is required otherwise our `modules.order` generated by `./build-linux` and installed with `BR2_ROOTFS_OVERLAY` overwrites the Buildroot generated one.
+As you have just seen, this sets up everything so that <<modprobe>> can conrrectly find the module.
 
 `./build-buildroot --build-linux` and `./run --buildroot-linux` are needed because the Buildroot kernel modules must use the Buildroot Linux kernel at build and run time.
 
+The `--no-overlay` is required otherwise our `modules.order` generated by `./build-linux` and installed with `BR2_ROOTFS_OVERLAY` overwrites the Buildroot generated one.
+
+Implementattion described at: https://stackoverflow.com/questions/40307328/how-to-add-a-linux-kernel-driver-module-as-a-buildroot-package/43874273#43874273
+
 ==== patches directory
 
 ===== patches/global

build-buildroot

@@ -46,6 +46,15 @@ Pass multiple times to use multiple fragment files.
             help='''\
 Don't build the all target which normally gets build by default.
 That target builds the root filesystem and all its dependencies.
+'''
+        )
+        parser.add_argument(
+            '--no-overlay', default=self._defaults['no_all'], action='store_true',
+            help='''\
+Don't add our overlay which contains all files we build without going through Buildroot.
+This prevents us from overwriting certain Buildroot files. Remember however that you must
+still rebuild the Buildroot package that provides those files to actually put the Buildroot
+files on the root filesystem.
 '''
         )
         parser.add_argument(
@@ -106,9 +115,6 @@ usually extra Buildroot targets.
                 'BR2_PACKAGE_OVERRIDE_FILE="{}"'.format(
                     self._path_relative_to_buildroot(os.path.join(common.root_dir, 'buildroot_override'))
                 ),
-                'BR2_ROOTFS_OVERLAY="{}"'.format(
-                    self._path_relative_to_buildroot(common.out_rootfs_overlay_dir),
-                ),
                 'BR2_ROOTFS_POST_BUILD_SCRIPT="{}"'.format(
                     self._path_relative_to_buildroot(os.path.join(common.root_dir, 'rootfs-post-build-script'))
                 ),
@@ -116,6 +122,10 @@ usually extra Buildroot targets.
                     self._path_relative_to_buildroot(os.path.join(common.root_dir, 'user_table'))
                 ),
             ])
+            if not args.no_overlay:
+                configs.append('BR2_ROOTFS_OVERLAY="{}"'.format(
+                    self._path_relative_to_buildroot(common.out_rootfs_overlay_dir)
+                ))
             config_fragments = [
                 os.path.join(common.root_dir, 'buildroot_config', 'default')
             ] + args.config_fragment

buildroot_packages/kernel_modules/Config.in

@@ -2,4 +2,4 @@ config BR2_PACKAGE_KERNEL_MODULES
 	bool "kernel_modules"
 	depends on BR2_LINUX_KERNEL
 	help
-		https://github.com/cirosantilli/linux-kernel-module-cheat#kernel_modules-package
+		See README.adoc

buildroot_packages/kernel_modules/README.adoc

@@ -0,0 +1 @@
+https://github.com/cirosantilli/linux-kernel-module-cheat#kernel_modules-package