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