android-info.txt: do not output 'board=' if no board_name

If no board name, output nothing to the android-info.txt.
Currently the output of 'board=' might trigger some parsing error
on some clients.

Bug: 189990989
Test: make out/target/product/generic_arm64/android-info.txt
Change-Id: Ia9aab6c092c07349aa57f0e73f8c69b6e05874c1

target/board/Android.mk

@@ -24,8 +24,10 @@ $(INSTALLED_ANDROID_INFO_TXT_TARGET): $(board_info_txt) build/make/tools/check_r
 	$(call pretty,"Generated: ($@)")
 ifdef board_info_txt
 	$(hide) grep -v '#' $< > $@
-else
+else ifdef TARGET_BOOTLOADER_BOARD_NAME
 	$(hide) echo "board=$(TARGET_BOOTLOADER_BOARD_NAME)" > $@
+else
+	$(hide) echo "" > $@
 endif
 
 # Copy compatibility metadata to the device.