make: fix buildsizes* targets

- also ignore BOARD_INSUFFICIENT_RAM boards
- unify/make work BOARD filter

Not sure why it failed I don't know the `<<<` construct .. I've got a `GNU bash, version 4.3.18(1)-release (x86_64-unknown-linux-gnu)`
Please use `" $${BOARDS} "` instead, my brain cannot not parse invisible characters. :)
And what's wrong with `"s/ $${BOARD} / /"`?