feat(esp8266): add system version

Bootloader can get the version of application for specific section,
and then check if some features are supported.

Developers can use the macro "ESP_IDF_VERSION" to limite some function like following:

include "esp_idf_version.h"

if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(3, 4, 0) && ESP_IDF_VERSION < ESP_IDF_VERSION_VAL(4, 0, 0) // 3.4 <= ver < 4.0
    do_xxx_process();
endif

components/esp8266/component.mk

@@ -34,6 +34,7 @@ COMPONENT_ADD_LDFLAGS += -L$(COMPONENT_PATH)/lib \
                          -T $(COMPONENT_BUILD_DIR)/esp8266.project.ld \
                          -Wl,--no-check-sections \
                          -u call_user_start      \
+                         -u g_esp_sys_info \
                          $(addprefix -T ,$(LINKER_SCRIPTS))
 
 ALL_LIB_FILES := $(patsubst %,$(COMPONENT_PATH)/lib/lib%.a,$(LIBS))