kdump-load: Add usage/help messages to this script

The kdump-load script is the only one that has more than a
single functionality, potentially being invoked in different ways.

With that in mind, just add a small usage/help for users'
information (thanks Emil for the suggestion).

While at it, changed the additional commands to be more friendly
and require exactly one command to be passed - changes summary:
s/initrd/create-initrd
s/clear/clear-initrd
s//load

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

00-default.conf

@@ -7,7 +7,7 @@
 #
 #  Configuration settings for kdump/pstore. After _any_ change in this
 #  file, it's required to re-create the kdump minimal initramfs by running:
-#  /usr/lib/kdump/kdump-load.sh initrd
+#  /usr/lib/kdump/kdump-load.sh create-initrd
 #
 #
 #  Pstore-RAM settings

kdump-init.service

@@ -10,7 +10,7 @@ Description=pstore/kdump loader boot-time service
 [Service]
 Type=oneshot
 StandardOutput=journal
-ExecStartPre=/usr/lib/kdump/kdump-load.sh
+ExecStartPre=/usr/lib/kdump/kdump-load.sh load
 ExecStart=/usr/lib/kdump/save-dumps.sh
 RemainAfterExit=yes
 

kdump-load.header

@@ -1,5 +1,5 @@
 #!/bin/bash
-set -uo pipefail
+set -o pipefail  #  Didn't set -u on purpose, since users could pass empty $1
 #
 #  SPDX-License-Identifier: LGPL-2.1+
 #

kdump-load.sh.in

@@ -71,21 +71,60 @@ cleanup_unused_initrd() {
 	rm -f "${INSTALLED_KERNELS}"
 }
 
+#  Now this routine performs a full deletion of all kdump initrd files.
+clear_all_initrds() {
+	rm -f "${MOUNT_FOLDER}"/kdump-initrd-*
+}
+
+#  Function to display basic help about how to use this tool.
+usage() {
+    cat <<EOF
+${0##*/} <COMMAND>
+Kdump/Pstore loader.
+
+Options:
+    load
+                        Load pstore/kdump according to the configuration file.
+
+    create-initrd
+                        Create the minimal kdump initrd for the running kernel.
+
+    clear-initrd
+                        Delete all kdump minimal initrd images.
+EOF
+}
+
+preamble() {
 	load_kdump_config
 
 	#  In case the kdump main folder doesn't exist, create it
 	#  here, as soon as possible.
 	mkdir -p "${MOUNT_FOLDER}"
+}
 
-if [ "$1" = "initrd" ]; then
+#  Entry point of the script.
+case $1 in
+	clear-initrd)
+		preamble
+		clear_all_initrds
+		exit 0
+		;;
+	create-initrd)
+		preamble
 		create_initrd
 		exit 0
-fi
+		;;
+	load)
+		#  just bail from the case statement, jumping to code below
+		;;
+	*)
+		usage
+		exit 1
+		;;
+esac
 
-if [ "$1" = "clear" ]; then
-	rm -f "${MOUNT_FOLDER}"/kdump-initrd-*
-	exit 0
-fi
+#  Here starts the main purpose of this script, the load operation.
+preamble
 
 #  Pstore-RAM load; if it is configured via the config files and fails
 #  to configure pstore, we still try to load the kdump. We try to reserve