re-add default under cplusplus instead

Author: mcspr

cores/esp8266/Esp.cpp

@@ -471,7 +471,7 @@ bool EspClass::checkFlashCRC() {
     uint32_t firstPart = (uintptr_t)&__crc_len - 0x40200000; // How many bytes to check before the 1st CRC val
 
     // Start the checksum
-    uint32_t crc = crc32((const void*)0x40200000, firstPart, 0xffffffff);
+    uint32_t crc = crc32((const void*)0x40200000, firstPart);
     // Pretend the 2 words of crc/len are zero to be idempotent
     crc = crc32(z, 8, crc);
     // Finish the CRC calculation over the rest of flash

cores/esp8266/core_esp8266_eboot_command.cpp

@@ -29,7 +29,7 @@ extern "C" {
 
 static uint32_t eboot_command_calculate_crc32(const struct eboot_command* cmd)
 {
-    return crc32((const uint8_t*) cmd, offsetof(struct eboot_command, crc32), 0xffffffff);
+    return crc32((const uint8_t*) cmd, offsetof(struct eboot_command, crc32));
 }
 
 int eboot_command_read(struct eboot_command* cmd)

cores/esp8266/coredecls.h

@@ -28,11 +28,12 @@ void __disableWiFiAtBootTime (void) __attribute__((noinline));
 void __real_system_restart_local() __attribute__((noreturn));
 
 uint32_t sqrt32(uint32_t n);
-uint32_t crc32(const void* data, size_t length, uint32_t crc);
 
 #ifdef __cplusplus
 }
 
+uint32_t crc32(const void* data, size_t length, uint32_t crc = 0xffffffff);
+
 #include <functional>
 
 using BoolCB = std::function<void(bool)>;