Add framework for configuring boot stack size

Add the target config option "boot-stack-size" which is passed to the
linker as the define "MBED_BOOT_STACK_SIZE" so the linker can
adjust the stack accordingly. On mbed 2 the boot stack becomes the
main stack after boot.  On mbed 5 the boot stack becomes the
ISR stack after boot. Because of these different uses  the stack size
for mbed 2 is set to 4K by default while on mbed 5 it is set to 1k.

Additionally, the NRF5X family requires a larger interrupt stack size
due to the softdevice so the size  is increased to 2k on mbed 5 builds.

rtos/mbed_lib.json

@@ -3,5 +3,22 @@
     "config": {
         "present": 1
     },
-    "macros": ["_RTE_"]
+    "macros": ["_RTE_"],
+    "target_overrides": {
+        "*": {
+            "target.boot-stack-size": "0x400"
+        },
+	    "MCU_NRF51": {
+	        "target.boot-stack-size": "0x800"
+	    },
+	    "MCU_NRF52840": {
+	        "target.boot-stack-size": "0x800"
+	    },
+	    "MCU_NRF52832": {
+	        "target.boot-stack-size": "0x800"
+	    },
+	    "MCU_NRF51_UNIFIED": {
+	        "target.boot-stack-size": "0x800"
+	    }
+    }
 }

targets/targets.json

@@ -20,6 +20,10 @@
             "network-default-interface-type": {
                 "help": "Default network interface type. Typical options: null, ETHERNET, WIFI, CELLULAR, MESH",
                 "value": null
+            },
+            "boot-stack-size": {
+                "help": "Define the boot stack size in bytes. This value must be a multiple of 8",
+                "value": "0x1000"
             }
         }
     },

tools/toolchains/__init__.py

@@ -742,12 +742,22 @@ class mbedToolchain:
         except ConfigException:
             pass
 
+    def add_linker_defines(self):
+        stack_param = "target.boot-stack-size"
+        params, _ = self.config_data
+
+        if stack_param in params:
+            define_string = self.make_ld_define("MBED_BOOT_STACK_SIZE", int(params[stack_param].value, 0))
+            self.ld.append(define_string)
+            self.flags["ld"].append(define_string)
+
     # Set the configuration data
     def set_config_data(self, config_data):
         self.config_data = config_data
         # new configuration data can change labels, so clear the cache
         self.labels = None
         self.add_regions()
+        self.add_linker_defines()
 
     # Creates the configuration header if needed:
     # - if there is no configuration data, "mbed_config.h" is not create (or deleted if it exists).