STM32G4: target directoty restructuration

- Alignment with other STM32 families
- Subfamily level in targets.json

Author: jeromecoutant

targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G431xx/TOOLCHAIN_ARM_STD/startup_stm32g431xx.S renamed to targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G431xB/TOOLCHAIN_ARM/startup_stm32g431xx.S

@@ -25,13 +25,13 @@
   #define MBED_APP_SIZE  MBED_ROM_SIZE
 #endif
 
-/* This value is normally defined by the tools to 0x1000 for bare metal and 0x400 for RTOS */
 #if !defined(MBED_CONF_TARGET_BOOT_STACK_SIZE)
-#  if defined(MBED_BOOT_STACK_SIZE)
-#    define MBED_CONF_TARGET_BOOT_STACK_SIZE MBED_BOOT_STACK_SIZE
-#  else
-#    define MBED_CONF_TARGET_BOOT_STACK_SIZE 0x400
-#  endif
+/* This value is normally defined by the tools to 0x1000 for bare metal and 0x400 for RTOS */
+#if defined(MBED_BOOT_STACK_SIZE)
+#define MBED_CONF_TARGET_BOOT_STACK_SIZE MBED_BOOT_STACK_SIZE
+#else
+#define MBED_CONF_TARGET_BOOT_STACK_SIZE 0x400
+#endif
 #endif
 
 /* Round up VECTORS_SIZE to 8 bytes */

targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G431xx/TOOLCHAIN_ARM_STD/stm32g431xx.sct renamed to targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G431xB/TOOLCHAIN_ARM/stm32g431xb.sct

@@ -32,7 +32,7 @@ if (!isdefinedsymbol(MBED_APP_SIZE)) {
 }
 
 if (!isdefinedsymbol(MBED_CONF_TARGET_BOOT_STACK_SIZE)) {
-    /* This value is normally defined by the tools 
+    /* This value is normally defined by the tools
         to 0x1000 for bare metal and 0x400 for RTOS */
     define symbol MBED_CONF_TARGET_BOOT_STACK_SIZE = 0x400;
 }