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.

Makefile

@@ -1843,7 +1843,7 @@ quiet_cmd_gen_envp = ENVP    $@
 			-I$(srctree)/arch/$(ARCH)/include \
 			$< -o $@; \
 	else \
-		echo -n >$@ ; \
+		touch $@ ; \
 	fi
 include/generated/env.in: include/generated/env.txt FORCE
 	$(call cmd,gen_envp)
@@ -1860,7 +1860,7 @@ quiet_cmd_envc = ENVC    $@
 	elif [ -n "$(ENV_SOURCE_FILE)" ]; then \
 		echo "Missing file $(ENV_FILE_CFG)"; \
 	else \
-		echo -n >$@ ; \
+		touch $@ ; \
 	fi
 
 include/generated/env.txt: $(wildcard $(ENV_FILE)) FORCE