Enable warning/error on missing return statement

Fixes esp8266#8475

We disable all errors for `None` mode, so manually re-enable the return-type
check as an error (because the code behavior is undefined and will crash
the core).

Author: earlephilhower

platform.txt

@@ -22,7 +22,7 @@ runtime.tools.cp={runtime.platform.path}/tools/cp.py
 runtime.tools.eboot={runtime.platform.path}/bootloaders/eboot/eboot.elf
 
 compiler.warning_flags=-w -Werror=return-type
-compiler.warning_flags.none=-w -Werror=return-type
+compiler.warning_flags.none=-w -Wreturn-type -Werror=return-type
 compiler.warning_flags.default=-Werror=return-type
 compiler.warning_flags.more=-Wall -Werror=return-type
 compiler.warning_flags.all=-Wall -Wextra -Werror=return-type