proxmox-boot: warn on missing systemd-boot package

proxmox-boot: warn on missing systemd-boot package

With the shipping of systemd-boot as separate package, we cannot rely
on `bootctl` being present in all systems (e.g. currently all systems
upgraded from PVE 7 will not automatically pull systemd-boot in.

This patch adds a check for existence + warning with an explanation to
the only invocation of bootctl in the boot-tool codebase

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>