sysboot: add a section on how to determine the used bootloader

Besides observing the boot process, `efibootmgr` seems to be the safest
bet. It should not show anything if booted in legacy BIOS mode. If
booted in UEFI mode, the boot entries should show which bootloader is
present.

Having both present should not happen in a regular use case AFAICT and
would need more consideration, or better, a reboot.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>

system-booting.adoc

@@ -12,6 +12,45 @@ For EFI Systems installed with ZFS as the root filesystem `systemd-boot` is
 used. All other deployments use the standard `grub` bootloader (this usually
 also applies to systems which are installed on top of Debian).
 
+[[sysboot_determine_bootloader_used]]
+Determine which bootloader is used
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+[thumbnail="screenshot/boot-grub.png", float="left"]
+
+The simplest and most reliable way to determine which bootloader is used, is to
+watch the boot process of the {pve} node.
+
+You will either see the blue box of `grub` or the simple black on white
+`systemd-boot`.
+
+[thumbnail="screenshot/boot-systemdboot.png"]
+
+Determining the bootloader from a running system might not be 100% accurate. The
+safest way is to run the following command:
+
+
+----
+# efibootmgr -v
+----
+
+If it returns a message that EFI variables are not supported, `grub` is used in
+BIOS/Legacy mode.
+
+If the output contains a line that looks similar to the following, `grub` is
+used in UEFI mode.
+
+----
+Boot0005* proxmox	[...] File(\EFI\proxmox\grubx64.efi)
+----
+
+If the output contains a line similar to the following, `systemd-bood` is used.
+
+----
+Boot0006* Linux Boot Manager	[...] File(\EFI\systemd\systemd-bootx64.efi)
+----
+
+
 [[sysboot_installer_part_scheme]]
 Partitioning scheme used by the installer
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~