Copy bootloader to PRODUCT_OUT

Implementation details:
- Import BOARD_PREBUILT_BOOTLOADER to Soong, and create a filegroup
  module
- Introduce `bootloader` property in android_device
- Introduce file copying (to PRODUCT_OUT) mechanism in android_device

Test: m aosp_cf_x86_64_phone_generated_device --soong-only
Bug: 388852084
Change-Id: I89a08590e39e771019e507b419fafae37bc4bb97
diff --git a/android/variable.go b/android/variable.go
index 6e46e9d..4b61827 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -697,6 +697,8 @@
 	TargetScreenDensity string `json:",omitempty"`
 
 	PrivateRecoveryUiProperties map[string]string `json:",omitempty"`
+
+	PrebuiltBootloader string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {