efi/libstub: add initial Boot Loader Interface support

The Boot Loader Interface (BLI) [1] defines EFI variables that expose
boot loader state to the running OS. LoaderInfo identifies the boot
loader, while LoaderDevicePartUUID records the GPT partition UUID of
the partition containing it.

LoaderDevicePartUUID is used, for example, by systemd-gpt-auto-generator
[2] to identify the disk the boot loader was launched from and
automatically detect and mount partitions on it.

GRUB [3] and systemd-boot [4] populate these variables, but when the
kernel is started directly by EFI firmware, there is no conventional
external boot loader to provide them. In that case, because the EFI stub
performs the boot loader role, it should provide the variables itself.

Use LoaderInfo as a sentinel: if it is already set by an earlier boot
stage or cannot be set, bail out. Otherwise, populate the other BLI
variables.

Parse the loaded image device path, extract the GUID signature from its
GPT HD() node and publish it under the Linux loader entry vendor GUID as
the volatile LoaderDevicePartUUID EFI variable.

Install the efi_bli_set_variables() hook in both the generic efi-stub.c
path and the x86-specific x86-stub.c path.

[1] The Boot Loader Interface
Link: https://systemd.io/BOOT_LOADER_INTERFACE/

[2] systemd-gpt-auto-generator
Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-gpt-auto-generator.html

[3] GRUB -- §16.2 bli
Link: https://www.gnu.org/software/grub/manual/grub/html_node/bli_005fmodule.html

[4] systemd -- systemd-boot UEFI Boot Manager
Link: https://github.com/systemd/systemd/blob/main/docs/BOOT.md?plain=1#L102

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
[ardb: - constify 'image' pointer parameter
       - pass efi_guid_t* to efi_snprintf()]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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