Make stack sizes configurable by the app

Application can overwrite stack sizes by defining main-stack-size and
thread-stack-size in mbed_app.json

Author: bulislaw

rtos/mbed_boot.c

@@ -178,10 +178,12 @@ WEAK void mbed_main(void);
 void pre_main (void);
 
 osThreadAttr_t _main_thread_attr;
-/* The main stack size is hardcoded on purpose, so it's less tempting to change it per platform. As usually it's not
- * the correct solution to the problem and it makes mbed OS behave differently on different targets.
- */
-MBED_ALIGN(8) char _main_stack[4096];
+
+/** The main thread's stack size can be configured by the application, if not explicitly specified it'll default to 4K */
+#ifndef MBED_CONF_APP_MAIN_STACK_SIZE
+#define MBED_CONF_APP_MAIN_STACK_SIZE 4096
+#endif
+MBED_ALIGN(8) char _main_stack[MBED_CONF_APP_MAIN_STACK_SIZE];
 mbed_rtos_storage_thread_t _main_obj;
 
 osMutexId_t               singleton_mutex_id;

rtos/rtx2/mbed_rtx_conf.h

@@ -24,14 +24,13 @@
 
 #include "mbed_rtx.h"
 
-#ifndef OS_STACK_SIZE
-#ifndef MBED_SMALL_TARGET
-#define OS_STACK_SIZE               4096
-#else
-#define OS_STACK_SIZE               2048
-#endif
+/** The thread's stack size can be configured by the application, if not explicitly specified it'll default to 4K */
+#ifndef MBED_CONF_APP_THREAD_STACK_SIZE
+#define MBED_CONF_APP_THREAD_STACK_SIZE 4096
 #endif
 
+#define OS_STACK_SIZE               MBED_CONF_APP_THREAD_STACK_SIZE
+
 #define OS_TIMER_THREAD_STACK_SIZE 768
 #define OS_IDLE_THREAD_STACK_SIZE  256