Fix the Makefile so WUHB gets built correctly

[gblues]

== DETAILS the parameters for content and icons are not organized correctly.

• the APP_ prefix is missing
• like the other APP_ flags, they need to be set before wut_rules is included

I've updated the sample makefile to correct this, as well as provide tips for how to build a path and what resolution the images need to be.

[fincs]

You might have missed this bit: https://github.com/devkitPro/wut/blob/master/samples/make/helloworld/Makefile#L109-L135