Autogenerate a system_dlkm android_filesystem

This .img file will package the .ko files returned by
`BOARD_SYSTEM_KERNEL_MODULES` (plus some other files). The .ko files
will be clustered by directory, and a `prebuilt_kernel_modules` will
be automatically created for each cluster. These will then be added to
the deps of the top-level system_dlkm android_filesystem.

Bug: 376726109
Test: m out/soong/.intermediates/build/soong/fsgen/soong_filesystem_creator/android_common/diff_test_aosp_cf_x86_64_phone_generated_system_dlkm_image.txt
KATI only installed file(s):
  etc/NOTICE.xml.gz
  etc/build.prop
  etc/fs_config_dirs
  etc/fs_config_files

Change-Id: I96669ad9595379c4de4c67331fc56bfa8d93e036
diff --git a/android/variable.go b/android/variable.go
index a4ee886..df9db7c 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -605,6 +605,9 @@
 	ProductLinkerConfigSrcs []string `json:",omitempty"`
 
 	ProductCopyFiles map[string]string `json:",omitempty"`
+
+	BuildingSystemDlkmImage bool     `json:",omitempty"`
+	SystemKernelModules     []string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {