Add support for kernel overlays

Core Kernel modules are now installed to
/usr/lib/kernel-overlays/base/lib/modules instead of /usr/lib/modules

Firmware files are installed to
/usr/lib/kernel-overlays/base/lib/firmware instead of /usr/lib/firmware

Packages can choose to install their modules and/or firmwareinto a
different overlay directory than base so they are not automatically enabled.

A new systemd service kernel-overlays, which is run before any modules
are loaded, adds overlays enabled via /storage/.cache/kernel-overlays/*.conf

These conf files must either contain the name of a system overlay,
which will then be searched for in /usr/lib/kernel-overlays/, or an
absolute path starting with '/'.

Overlaying is implemented by symlinking files from the overlays
to /usr/lib/modules and /usr/lib/firmware as not all platforms
support overlayfs.

This overlaying system can also be used by addons to provide optional
kernel modules and/or firmware by storing the absolute path to the addon
directory in the conf file and shipping kernel modules in
<addondir>/lib/modules/<KERNELVER>.

To save space only symlinks for modules matching the currently
running kernel version are created as addons may want to ship
modules for older kernel versions as well to be backward-compatible
to older LibreELEC versions.

Changes since v1:
- fixup kernel-overlays-service: add before kmod-static-nodes.service
  Fix static device nodes for module autoloading not being created.
- fixup kernel-overlays-service: fail if /usr/lib/modules exists
- squash kernel overlays: use get_kernel_overlay_dir in linux/package.mk
- Add support for firmware files via kernel overlays
- config/functions: include kernel overlays in addon installation

Signed-off-by: Matthias Reichl <hias@horus.com>

packages/sysutils/busybox/scripts/init

@@ -1011,7 +1011,7 @@
     mount --move /flash /sysroot/flash
     mount --move /storage /sysroot/storage
 
-    if [ ! -d "/sysroot/usr/lib/modules/$(uname -r)/" -a -f "/sysroot/usr/lib/systemd/systemd" ]; then
+    if [ ! -d "/sysroot/usr/lib/kernel-overlays/base/lib/modules/$(uname -r)/" -a -f "/sysroot/usr/lib/systemd/systemd" ]; then
       echo ""
       echo "NEVER TOUCH boot= in extlinux.conf / cmdline.txt!"
       echo "If you don't know what you are doing,"