Reverted -Wall

the current Arduino platform has warnings disabled by default.
There are ongoing development upstream to implement all-warning
from preferences.

platform.txt

@@ -11,12 +11,12 @@ version=1.8.0
 
 compiler.path={runtime.ide.path}/hardware/tools/gcc-arm-none-eabi-4.8.3-2014q1/bin/
 compiler.c.cmd=arm-none-eabi-gcc
-compiler.c.flags=-mcpu={build.mcu} -mthumb -c -g -Os -Wall -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500
+compiler.c.flags=-mcpu={build.mcu} -mthumb -c -g -Os -W -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500
 compiler.c.elf.cmd=arm-none-eabi-gcc
 compiler.c.elf.flags=-Os -Wl,--gc-sections -save-temps
 compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=arm-none-eabi-g++
-compiler.cpp.flags=-mcpu={build.mcu} -mthumb -c -g -Os -Wall -ffunction-sections -fdata-sections -fno-threadsafe-statics -nostdlib --param max-inline-insns-single=500 -fno-rtti -fno-exceptions
+compiler.cpp.flags=-mcpu={build.mcu} -mthumb -c -g -Os -W -ffunction-sections -fdata-sections -fno-threadsafe-statics -nostdlib --param max-inline-insns-single=500 -fno-rtti -fno-exceptions
 compiler.ar.cmd=arm-none-eabi-ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=arm-none-eabi-objcopy