Fix APP_PLATFORM not properly passed in NDKRecipe

[ghost]

Fix APP_PLATFORM not properly passed in NDKRecipe. This is needed so certain NDKAPI/--minimum-api values won't lead to build errors with NDK based recipes, e.g. when using NDKAPI=21 with the current jpeg recipe

[ghost]

Ok tested & ready for merge! it doesn't actually fix jpeg for NDKAPI 21 (seems to be a fundamental issue with the android libjpeg-turbo fork) but it should fix others, and I verified that the APP_PLATFORM env var is set, and recognized by the make script without a warning that it would be some invalid or unknown platform

[ghost]

The additional pushes you see are just rebases to latest master. I used this during a couple of builds now without any issues 👍 (all python 3 though, no python 2 builds)

[inclement]

Thanks