SPIFFS libc integration refactoring; ESP32 support

Factor out common part of SPIFFS libc interface support from the CC3200
port and reuse it for ESP32. Minor cleanups.

PUBLISHED_FROM=19ad4049372f83046abd8eba29cfc7c2cc405671

mongoose.h

@@ -44,6 +44,7 @@
 #define CS_P_CUSTOM 0
 #define CS_P_UNIX 1
 #define CS_P_WINDOWS 2
+#define CS_P_ESP32 15
 #define CS_P_ESP8266 3
 #define CS_P_CC3200 4
 #define CS_P_MSP432 5
@@ -56,7 +57,7 @@
 #define CS_P_NRF51 12
 #define CS_P_NRF52 10
 #define CS_P_PIC32 11
-/* Next id: 15 */
+/* Next id: 16 */
 
 /* If not specified explicitly, we guess platform by defines. */
 #ifndef CS_PLATFORM
@@ -79,6 +80,8 @@
 #define CS_PLATFORM CS_P_NXP_KINETIS
 #elif defined(PIC32)
 #define CS_PLATFORM CS_P_PIC32
+#elif defined(ESP_PLATFORM)
+#define CS_PLATFORM CS_P_ESP32
 #elif defined(ICACHE_FLASH)
 #define CS_PLATFORM CS_P_ESP8266
 #elif defined(TARGET_IS_TM4C129_RA0) || defined(TARGET_IS_TM4C129_RA1) || \
@@ -103,6 +106,7 @@
 
 /* Amalgamated: #include "common/platforms/platform_unix.h" */
 /* Amalgamated: #include "common/platforms/platform_windows.h" */
+/* Amalgamated: #include "common/platforms/platform_esp32.h" */
 /* Amalgamated: #include "common/platforms/platform_esp8266.h" */
 /* Amalgamated: #include "common/platforms/platform_cc3200.h" */
 /* Amalgamated: #include "common/platforms/platform_cc3100.h" */