metropolis: implement and use A/B preloader

This switches over from using the EFI built-in bootloader for A/B
updates to using our own EFI preloader due to significant issues with
in-the-wild EFI implementations.  It is a very minimal design relying
on a single Protobuf state file instead of EFI variables.

Change-Id: Ieebd0a8172ebe3f44c69b3e8c278c53d3fe2eeb4
Reviewed-on: https://review.monogon.dev/c/monogon/+/2203
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/installer/main.go b/metropolis/installer/main.go
index 5481c3f..8aa8836 100644
--- a/metropolis/installer/main.go
+++ b/metropolis/installer/main.go
@@ -21,6 +21,8 @@
 
 import (
 	"archive/zip"
+	"bytes"
+	_ "embed"
 	"errors"
 	"fmt"
 	"io/fs"
@@ -38,6 +40,9 @@
 	"source.monogon.dev/metropolis/pkg/sysfs"
 )
 
+//go:embed metropolis/node/core/abloader/abloader_bin.efi
+var abloader []byte
+
 const mib = 1024 * 1024
 
 // mountPseudoFS mounts efivarfs, devtmpfs and sysfs, used by the installer in
@@ -228,6 +233,7 @@
 		},
 		SystemImage:    systemImage,
 		EFIPayload:     FileSizedReader{efiPayload},
+		ABLoader:       bytes.NewReader(abloader),
 		NodeParameters: FileSizedReader{nodeParameters},
 	}
 	// Calculate the minimum target size based on the installation parameters.