Description

Environment variables that are set after the inclusion of Makefile.include in an application's Makefile seem to be ignored. This results in e.g. the default channel not to be configurable via environment variables in gnrc_networking

Steps to reproduce the issue

Try to compile the gnrc_networking example with DEFAULT_CHANNEL set to a different value than the actual default.

Expected results

The channel should be changed

Actual results

The channel is the same

Versions

a0a3112 of RIOT.