boot-tool: disarm upstream systemd-boot hookscripts

boot-tool: disarm upstream systemd-boot hookscripts

With Debian Bookworm systemd-boot is a separate binary-package,
instead of part of the main systemd package.
Since it's not installed by default, Debian-upstream has added
hook-scripts to the package, which manage kernel copying to the esp
(kernel-install).

The hookscripts print a warning if the ESP is not mounted at
$SYSTEMD_ESP_PATH or /boot/efi, /efi or /boot - through `bootctl
is-installed --quiet` [0,1].

This patch adds a function, which disables the hookscripts from
upstream if /etc/kernel/proxmox-boot-uuids is present.
It adds an explanation as marker and 'exit 0' on top of the script, so
that users know why the scripts were touched (e.g. when a new
systemd-boot hookscript version from upstream asks what to do with the
local modifications)

While editing shell-script hooks from other packages is quite brittle
it still seems like the best option, to support most use-cases
(including users, who don't use proxmox-boot-tool, but want to
manually install systemd-boot).
Alternatives considered:
* dpkg-divert for all hookscripts - sadly the Debian policy manual
  warns against this
* adding Replaces: systemd-boot to d/control - afaict this would need
  systemd-boot to also declare this for proxmox-kernel-helper [3]

Tested on 2 VMs installed with the 8.0 ISO (once with legacy once with
uefi boot)

[0]
https://github.com/systemd/systemd/blob/8a38b62f37189b071a30f208530ce5dc278e521e/src/shared/find-esp.c#L503
[1]
https://github.com/systemd/systemd/blob/8a38b62f37189b071a30f208530ce5dc278e521e/src/boot/bootctl.c#L90
[2] https://www.debian.org/doc/debian-policy/ap-pkg-diversions.html
[3] https://www.debian.org/doc/debian-policy/ch-relationships.html

Reported-by: Aaron Lauterer <a.lauterer@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
 [T: avoid potential false exit code, use if ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>