fix(esp8266): Fix compile script global varible error

Author: donghengqaz

components/esp8266/Makefile.projbuild

@@ -91,8 +91,13 @@ OTA2_BIN := ./build/$(PROJECT_NAME).app2.bin
 
 OTA_V2_TO_V3_BIN  := ./build/$(PROJECT_NAME).v2_to_v3.ota.bin
 
-CONFIG_APP2_OFFSET ?= $(CONFIG_APP1_OFFSET)
-CONFIG_APP2_SIZE ?= $(CONFIG_APP1_SIZE)
+ifndef CONFIG_APP2_OFFSET
+CONFIG_APP2_OFFSET := $(CONFIG_APP1_OFFSET)
+endif
+
+ifndef CONFIG_APP2_SIZE
+CONFIG_APP2_SIZE := $(CONFIG_APP1_SIZE)
+endif
 
 OTA1_OFFSET := $(CONFIG_APP1_OFFSET)
 ifdef CONFIG_ESP8266_BOOT_COPY_APP