Fix dump-products.

m dump-products didn't print anything useful prior to this change,
however this was visible in the terminal:
15:18:58 Error dumping make vars: Failed to parse make line: "==== build/target/product/aosp_arm.mk ===="

Convert this goal to normal target instead, which apart from making
$(info) actually print something, considers the build successful
when running "m dump-products" too. This makes it easier to run with
e.g. multiproduct_kati.

Test: m dump-products
Change-Id: I944c87cd0fd323687690b0fcc3c37a8f37242cd9