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.
diff --git a/esp8266/esp8266_common.ld b/esp8266/esp8266_common.ld
index 1da8356..bc983df 100644
--- a/esp8266/esp8266_common.ld
+++ b/esp8266/esp8266_common.ld
@@ -19,6 +19,8 @@
 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: */