Generate META/fastboot-info.txt file

Make packaging system generates META/fastboot-info.txt by
1. A checked-in fastboot-info file (TARGET_BOARD_FASTBOOT_INFO_FILE), OR
2. Generating a fastboot-info file from board config vars

This CL implements (1). Cuttlefish uses a checked-in fastboot-info.txt
file.

To implement this, a new property has been added to `android_device`

Test: Built Soong's META/fastboot-info.txt file locally
Bug: 399788523
Change-Id: Iadef020b1144ec72efc3bc3c7020fe63b6c47d4a
diff --git a/android/variable.go b/android/variable.go
index 7ac9f08..5bc0b29 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -716,6 +716,8 @@
 	ProductFsCompression string `json:",omitempty"`
 
 	ReleaseToolsExtensionDir string `json:",omitempty"`
+
+	BoardFastbootInfoFile string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {