feat: Added how to clean up efi entries

asciidoc/tips/metal3.adoc

@@ -205,3 +205,32 @@ spec:
 ====
 Kubernetes namespaces can be also used to better organize the different objects.
 ====
+
+== Clean up old EFI boot entries
+
+Sometimes, the https://en.wikipedia.org/wiki/UEFI#UEFI_booting[UEFI boot manager] contains multiple entries for older operating systems that are probably not needed anymore (especially for host being re-provisioned multiple times).
+You can clean up those old entries by following any of the following procedures:
+
+* Delete them on the BIOS/EFI setup interface directly (the exact procedure will depend on the hardware).
+* Run the UEFI https://uefi.org/sites/default/files/resources/UEFI_Shell_2_2.pdf[`bcfg`] shell as:
++
+----
+# List the entries
+bcfg boot dump -b
+# Delete entry number X
+bcfg boot rm X
+# X is the number associated the entry to remove. For example, if the entry is "Boot0002 foobar", then X is 2.
+----
+
+* Use `efibootmgr` on a Linux system as:
++
+----
+# List the entries
+efibootmgr -v
+# Delete entry number X
+efibootmgr -b X -B 
+----
+
+The process may leave orphaned files on the EFI System Parition (ESP), usually found under subdirectories named by the vendor (e.g., `EFI/opensuse` or `EFI/Microsoft`).
+While these files are generally harmless, they should be deleted if they consume excessive space as it can prevent the installation of a new OS or a boot manager update.
+Removal may require explicitly mounting the ESP, typically mounted as `/boot/efi/EFI` on Linux systems.