Ambiq_Micro: Workaround config passing bug

Workaround a bug where the boot stack size configuration option is not
passed on to armlink, the Arm Compiler's linker. Prefer
MBED_CONF_TARGET_BOOT_STACK_SIZE if present, as this is what the
configuration system should provide. Fall back to MBED_BOOT_STACK_SIZE
if MBED_CONF_TARGET_BOOT_STACK_SIZE is not defined, as in the case of
buggy tools. If both MBED_CONF_TARGET_BOOT_STACK_SIZE and
MBED_BOOT_STACK_SIZE are not defined, then we fall back to a hard-coded
value provided by the linkerscript. See
#13474 for more information.

Author: hugueskamba

targets/TARGET_Ambiq_Micro/TARGET_Apollo3/TOOLCHAIN_ARM_STD/AMA3B1KK.sct

@@ -23,6 +23,14 @@
 ; SOFTWARE.
 ; 
 
+#if !defined(MBED_CONF_TARGET_BOOT_STACK_SIZE)
+  /* 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
 
 #define MBED_APP_START 0x0000C000
 #define MBED_APP_SIZE 0x000F4000
@@ -32,8 +40,8 @@
 #define MBED_RAM0_SIZE  0x100
 #define MBED_RAM1_START (MBED_RAM0_START + MBED_RAM0_SIZE)
 #define MBED_RAM1_SIZE  (MBED_RAM_SIZE - (MBED_RAM0_SIZE))
-#define Stack_Size MBED_BOOT_STACK_SIZE
-#define RAM_FIXED_SIZE   (MBED_BOOT_STACK_SIZE+MBED_RAM0_SIZE)
+#define Stack_Size MBED_CONF_TARGET_BOOT_STACK_SIZE
+#define RAM_FIXED_SIZE   (MBED_CONF_TARGET_BOOT_STACK_SIZE+MBED_RAM0_SIZE)
 
 
 LR_IROM1 MBED_APP_START MBED_APP_SIZE {