Replace echo -n's used in environment processing with touch

echo -n does not give the intended effect when invoked in macOS through
/bin/sh, which is the shell make uses by default, see
"https://stackoverflow.com/questions/11675070/makefile-echo-n-not-working"
for a detailed explanation. In this case, it resulted in "-n" being
written to env.txt and env.in even though they should be empty, which
caused compilation to fail with "Your board uses a text-file
environment, so must not define CONFIG_EXTRA_ENV_SETTINGS".

This patch prevents the error by replacing echo -n's with touch, as they
are used to create empty files in these cases.