NRF52: Fix failure if boot stack size is not provided

The same default value is provided in the GCC_ARM linker file.

Author: hugueskamba

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52832/device/TOOLCHAIN_ARM_STD/nRF52832.sct

@@ -14,6 +14,10 @@
   #define MBED_RAM_SIZE 0x10000
 #endif
 
+#if !defined(MBED_CONF_TARGET_BOOT_STACK_SIZE)
+  #define MBED_CONF_TARGET_BOOT_STACK_SIZE 0x800
+#endif
+
 #define Stack_Size MBED_CONF_TARGET_BOOT_STACK_SIZE
 
 #define MBED_RAM0_START MBED_RAM_START

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/device/TOOLCHAIN_ARM_STD/nRF52840.sct

@@ -14,6 +14,10 @@
   #define MBED_RAM_SIZE 0x40000
 #endif
 
+#if !defined(MBED_CONF_TARGET_BOOT_STACK_SIZE)
+  #define MBED_CONF_TARGET_BOOT_STACK_SIZE 0x800
+#endif
+
 #define Stack_Size MBED_CONF_TARGET_BOOT_STACK_SIZE
 
 #define MBED_RAM0_START MBED_RAM_START