Enable warning for "double" type promotion

Author: ppescher

platform.txt

@@ -16,7 +16,7 @@ compiler.stm.extra_include="-I{build.source.path}" "-I{build.core.path}/avr" "-I
 compiler.warning_flags=-w
 compiler.warning_flags.none=-w
 compiler.warning_flags.default=
-compiler.warning_flags.more=-Wall
+compiler.warning_flags.more=-Wall -Wdouble-promotion
 compiler.warning_flags.all=-Wall -Wextra -Wdouble-promotion
 
 compiler.path={runtime.tools.arm-none-eabi-gcc-6-2017-q2-update.path}/bin/