Export TARGET_BOARD_FASTBOOT_INFO_FILE to soong

If set, this will be copied to target_file.zip as fastboot-info.txt

Test: presbumits
Bug: 399788523
Change-Id: I9279aafef10c5d97126b045fc8fdfbd0564e60c0
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 6590afa..c3cbaf3 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -569,6 +569,9 @@
 
   $(call add_json_str, ReleaseToolsExtensionDir, $(firstword $(TARGET_RELEASETOOLS_EXTENSIONS) $($(TARGET_DEVICE_DIR)/../common)))
 
+  # Fastboot
+  $(call add_json_str, BoardFastbootInfoFile, $(TARGET_BOARD_FASTBOOT_INFO_FILE))
+
 $(call end_json_map)
 
 # For converting vintf_data