[kernel][arm] set default stack size to 1K for cortex-m based devices

arch/arm/rules.mk

@@ -127,6 +127,10 @@ OBJS += \
 
 INCLUDES += \
 	-I$(LOCAL_DIR)/arm-m/CMSIS/CM3/CoreSupport
+
+# set a much smaller stack size
+DEFINES += \
+	DEFAULT_STACK_SIZE=1024
 endif
 
 # set the default toolchain to arm elf and set a #define

include/kernel/thread.h

@@ -93,7 +93,9 @@ typedef struct thread {
 #define HIGH_PRIORITY ((NUM_PRIORITIES / 4) * 3)
 
 /* stack size */
+#ifndef DEFAULT_STACK_SIZE
 #define DEFAULT_STACK_SIZE 8192
+#endif
 
 /* functions */
 void thread_init_early(void);