only auto-generate board= requirement if a board-info file is not specified

If a board-info file is specified (as is usually the case) we should honor
its requirements.
diff --git a/target/board/Android.mk b/target/board/Android.mk
index fc2f651..7137082 100644
--- a/target/board/Android.mk
+++ b/target/board/Android.mk
@@ -45,7 +45,8 @@
 board_info_txt := $(wildcard $(TARGET_DEVICE_DIR)/board-info.txt)
 $(INSTALLED_ANDROID_INFO_TXT_TARGET): $(board_info_txt)
 	$(call pretty,"Generated: ($@)")
-	$(hide) echo "board=$(TARGET_BOOTLOADER_BOARD_NAME)" > $@
 ifdef board_info_txt
 	$(hide) cat $< >> $@
+else
+	$(hide) echo "board=$(TARGET_BOOTLOADER_BOARD_NAME)" > $@
 endif