build: allow var.% targets to bypass the prepare steps

build: allow var.% targets to bypass the prepare steps

These targets are used to input variable values from the Make
context into other things like python scripts, so log messages
should be silenced and build prerequisites should be skipped.

The same thing is done for the other variable print target "val.%".

While at it, combine identical target rules into one definition.

Signed-off-by: Michael Pratt <mcpratt@pm.me>