Soong generates kernel_version_for_uffd_gc.txt

In the context of support "--soong-only", kernel_version_for_uffd_gc.txt
needs to be generated by Soong.

Soong will write the BOARD_KERNEL_VERSION to the txt file
in soong-only mode. It will panic if BOARD_KERNEL_VERSION
is not set.

Test: CI
Bug: 383564057
Change-Id: Idb264eaa91424ed9bab5be4988c7eb96334153f5
diff --git a/android/variable.go b/android/variable.go
index 3cf66ea..a60cad5 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -532,7 +532,8 @@
 	OdmPropFiles       []string `json:",omitempty"`
 	VendorPropFiles    []string `json:",omitempty"`
 
-	EnableUffdGc *string `json:",omitempty"`
+	EnableUffdGc       *string `json:",omitempty"`
+	BoardKernelVersion *string `json:",omitempty"`
 
 	BoardAvbEnable                         *bool    `json:",omitempty"`
 	BoardAvbSystemAddHashtreeFooterArgs    []string `json:",omitempty"`