stages/rpm: add new kernel_install_env options

Add support for setting environment variables for kernel-install and its
plugins.  These can be used to control the behaviour of kernel-install
during the kernel rpm installation.  Currently, we only add support for
$BOOT_ROOT, which we need to control the path where the kernel should be
installed.

Normally, kernel-install scripts will detect the correct path based on
the filesystem layout, specifically the ESP path.  However, during our
installation the filesystem is not yet set up, so with this option, we
can control the installation path based on the location we know the ESP
will be mounted in the final image.

This is a very similar override to what we do with the
org.osbuild.fix-bls stage.

The stage option is added under a kernel_install_env object so we can
potentially support more of the known env vars in the future.

stages/org.osbuild.rpm

@@ -202,6 +202,14 @@ def main(tree, inputs, options):
                 "--define", macro,
             ]
 
+        env = os.environ.copy()
+        # update the environment with variables for kernel-install if any are defined
+        kernel_install_env = options.get("kernel_install_env", {})
+        if kernel_install_env:
+            boot_root = kernel_install_env.get("boot_root")
+            if boot_root:
+                env["BOOT_ROOT"] = boot_root
+
         with tempfile.NamedTemporaryFile(prefix="manifest.", mode='w') as manifest:
             manifest.writelines(c + '\n' for c in packages)
             manifest.flush()
@@ -216,7 +224,7 @@ def main(tree, inputs, options):
                 # reading.
                 "--nosignature",
                 "--install", manifest.name
-            ], cwd=pkgpath, check=True)
+            ], cwd=pkgpath, check=True, env=env)
 
         for key in options.get("gpgkeys.fromtree", []):
             path = os.path.join(tree, key.lstrip("/"))

stages/org.osbuild.rpm.meta.json

@@ -146,6 +146,18 @@
         "ostree_booted": {
           "type": "boolean",
           "description": "Create the '/run/ostree-booted' marker"
+        },
+        "kernel_install_env": {
+          "description": "Set environment variables understood by kernel-install and plugins (kernel-install(8))",
+          "type": "object",
+          "additionalProperties": false,
+          "properties": {
+            "boot_root": {
+              "type": "string",
+              "pattern": "^\\/?(?!\\.\\.)((?!\\/\\.\\.\\/).)+$",
+              "description": "Sets $BOOT_ROOT for kernel-install to override $KERNEL_INSTALL_BOOT_ROOT, the installation location for boot entries"
+            }
+          }
         }
       }
     },