bli: Add a module for the Boot Loader Interface

Add a new module named bli. It implements a small but quite useful part
of the Boot Loader Interface [0]. This interface uses EFI variables for
communication between the boot loader and the operating system.

When loaded, this module sets two EFI variables under the vendor GUID
4a67b082-0a4c-41cf-b6c7-440b29bb8c4f:

- LoaderInfo: contains GRUB + <version number>.
  This allows the running operating system to identify the boot loader
  used during boot.

- LoaderDevicePartUUID: contains the partition UUID of the EFI System
  Partition (ESP). This is used by systemd-gpt-auto-generator [1] to
  find the root partitions (and others too), via partition type IDs [2].

This module is available on EFI platforms only. The bli module relies on
the part_gpt module which has to be loaded beforehand to make the GPT
partitions discoverable.

Update the documentation, add a new chapter "Modules" and describe the
bli module there.

[0] https://systemd.io/BOOT_LOADER_INTERFACE/
[1] https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html
[2] https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -96,6 +96,7 @@ This edition documents version @value{VERSION}.
 * Filesystem::                  Filesystem syntax and semantics
 * Interface::                   The menu and the command-line
 * Environment::                 GRUB environment variables
+* Modules::                     Available modules
 * Commands::                    Available builtin commands
 * Internationalisation::        Topics relating to language support
 * Security::                    Authentication, authorisation, and signatures
@@ -3867,6 +3868,28 @@ using BIOS or EFI functions (no ATA, USB or IEEE1275).
 @command{grub-mkconfig} uses this facility to implement
 @samp{GRUB_SAVEDEFAULT} (@pxref{Simple configuration}).
 
+@node Modules
+@chapter Modules
+
+In this chapter, we list all modules that are available in GRUB (currently incomplete).
+
+Modules can be loaded via the @command{insmod} (@pxref{insmod}) command.
+
+@menu
+* bli::
+@end menu
+
+@node bli
+@section bli
+This module provides basic support for the Boot Loader Interface. The Boot
+Loader Interface specifies a set of EFI variables that are used to communicate
+boot-time information between the boot loader and the operating system.
+
+When loaded, the bli module publishes the GPT partition UUID of the EFI System
+Partition used during boot and a string identifying GRUB as the active boot
+loader, including the version number.
+
+This module is only available on UEFI platforms.
 
 @node Commands
 @chapter Available commands