Record loaded kernel modules when hostonly mode is enabled

A hostonly image will not include every possibly required kernel module,
so if any hardware or configuration changed, the image may fail to boot.

One way to know if there are any hardware change or configuration change
that will require an image rebuild or not is to check the loaded kernel
module list. If the loaded kernel module list differs from last build
time, then the image may require to be rebuilt.

This commit will let dracut record the loaded kernel module list when
the image is being built, so other tools or services can compare this
list with currently loaded kernel modules to decide if dracut should be
called to rebuild the image.

To retrieve the loaded kernel modules list when an image is built, use
lsinitrd command:

lsinitrd $image -f */lib/dracut/loaded-kernel-modules.txt

dracut-functions.sh

@@ -676,6 +676,17 @@ get_ucode_file ()
     fi
 }
 
+# Get currently loaded modules
+# sorted, and delimited by newline
+get_loaded_kernel_modules ()
+{
+    local modules=( )
+    while read _module _size _used _used_by; do
+        modules+=( "$_module" )
+    done <<< $(lsmod | sed -n '1!p')
+    printf '%s\n' "${modules[@]}" | sort
+}
+
 # Not every device in /dev/mapper should be examined.
 # If it is an LVM device, touch only devices which have /dev/VG/LV symlink.
 lvm_internal_dev() {

dracut.sh

@@ -1492,6 +1492,9 @@ dinfo "*** Including modules done ***"
 
 ## final stuff that has to happen
 if [[ $no_kernel != yes ]]; then
+    if [[ $hostonly ]]; then
+        echo "$(get_loaded_kernel_modules)" > $initdir/lib/dracut/loaded-kernel-modules.txt
+    fi
 
     if [[ $drivers ]]; then
         hostonly='' instmods $drivers