esp8266/modesp: flash_user_start: Use value from linker script.

Make esp8266_common.ld export size reserved to all code segments, and use
that in esp.flash_user_start() implementation.

esp8266/esp8266_common.ld

@@ -19,6 +19,8 @@ EXTERN(_KernelExceptionVector)
 EXTERN(_NMIExceptionVector)
 EXTERN(_UserExceptionVector)
 
+_firmware_size = ORIGIN(irom0_0_seg) + LENGTH(irom0_0_seg) - 0x40200000;
+
 PROVIDE(_memmap_vecbase_reset = 0x40000000);
 
 /* Various memory-map dependent cache attribute settings: */

esp8266/modesp.c

@@ -159,12 +159,10 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_0(esp_flash_size_obj, esp_flash_size);
 // we assume there's a yaota8266 bootloader.
 #define IS_OTA_FIRMWARE() ((*(uint32_t*)0x40200000 & 0xff00) == 0x100)
 
+extern byte _firmware_size[];
+
 STATIC mp_obj_t esp_flash_user_start(void) {
-    if (IS_OTA_FIRMWARE()) {
-        return MP_OBJ_NEW_SMALL_INT(0x3c000 + 0x90000);
-    } else {
-        return MP_OBJ_NEW_SMALL_INT(0x90000);
-    }
+    return MP_OBJ_NEW_SMALL_INT((uint32_t)_firmware_size);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(esp_flash_user_start_obj, esp_flash_user_start);