all: Refactor the whole kdump/pstore folder setting

This is somewhat a big refactor. The early design of kdump/pstore was
meant to support the A/B scheme of Steam Deck and a dracut-based initrd
only. In this scheme, we had a DEVNODE (like nvme partition or a devlink)
that represented the device node to be mounted early in boot when kdump
was in use. Also, we had a folder defined in the config file on top of
such dev node, and a temporary file used to "carry" the composition of
the full kdump path across boot time scripts. Yeah, kinda complex setup.

We are now moving to a more generic approach, and for that, the design
choice was a more convenient/simple one for the common cases, that
requires some operations to properly work on SteamOS dracut-based initrd.
Now we have only a single path on config file, which should be accessible
in a R/W filesystem by both scripts executed in the systemd service. No
devnode information or temp file is used anymore.

But with that, comes the need of discovering the proper devnode and base
folder for kdump'ing early in boot, from the initrd. Using the findmnt
tool we manage to derive all the necessary data during the initrd
preparation phase. Also, while at it we manage to fix an "inconsistency"
of our dracut initrd creation script: installkernel() should be responsible
to deal with DRM modules removal, not install().

On top of this (already big) change, now our dracut initrd excludes not
only amdgpu driver/FWs, but radeon, nvidia and i915 as well. And due to
our refactor of the mount point information (using findmnt to collect info
during dracut initrd creation), we also allow now arbitrary filesystem
drivers to be included, i.e., we don't hardcode/limit for ext4 only.

Again, mea culpa for not splitting this in multiple atomic/simple commits,
the burden to keep a pretty git log is starting to consume precious time.

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

00-default.conf

@@ -22,17 +22,12 @@ PSTORE_RECORD_SZ=2097152
 #
 #
 #  Mount-related options
-#  MOUNT_DEVNODE is the desired mount-point unique link in the /dev/disk
-#  directory link; this is used to derive the numerical devnode for kdump,
 #  since the link is not present so early in the system boot. MOUNT_FOLDER
-#  should live in the filesystem of the devnode  (and will be created if
-#  doesn't exist); this is the place in which both pstore and kdump logs
-#  will be stored, as well as the kdump initrd and some ancillary data.
-#  Finally, MNT_TMP is just a temporary file that carries the full path
-#  of the mounted folder across boot-time scripts.
-MOUNT_DEVNODE="/dev/disk/by-partsets/shared/home"
-MOUNT_FOLDER="/.steamos/offload/var/kdump"
-MNT_TMP="/tmp/.kdump_load.mnt"
+#  MOUNT_FOLDER is the the place in which both pstore and kdump logs will
+#  be stored, as well as the kdump initrd and some ancillary data. This
+#  directory should be in an accessible filesystem (read/write) and if such
+#  folder doesn't exist, it'll be created.
+MOUNT_FOLDER="/home/.steamos/offload/var/kdump"
 #
 #
 #  Kdump controlling settings