cpu/esp_common: added for common files for ESP SoC

cpu/esp32/Makefile

@@ -2,6 +2,7 @@
 MODULE = cpu
 
 # Add a list of subdirectories, that should also be built:
+DIRS += $(RIOTCPU)/esp_common
 DIRS += periph
 DIRS += freertos
 DIRS += vendor
@@ -14,10 +15,6 @@ ifneq (, $(filter esp_eth, $(USEMODULE)))
     DIRS += esp-eth
 endif
 
-ifneq (, $(filter esp_now, $(USEMODULE)))
-    DIRS += esp-now
-endif
-
 ifneq (, $(filter esp_wifi, $(USEMODULE)))
     DIRS += esp-wifi
 endif

cpu/esp8266/Makefile

@@ -2,6 +2,7 @@
 MODULE = cpu
 
 # Add a list of subdirectories, that should also be built:
+DIRS += $(RIOTCPU)/esp_common
 DIRS += periph
 DIRS += sdk
 DIRS += vendor

cpu/esp_common/Makefile

+# add a list of subdirectories, that should also be build
+ifneq (, $(filter esp_now, $(USEMODULE)))
+    DIRS += esp-now
+endif
+
+include $(RIOTBASE)/Makefile.base

cpu/esp_common/doc.txt

+/**
+ * @defgroup    cpu_esp_common ESP Common
+ * @brief       Common modules for Espressif ESP SoCs
+ * @ingroup     cpu
+ */