components/spi_flash: make spi_flash APIs dual-core-compatible

See esp_spi_flash.c for implementation notes.

components/spi_flash/include/esp_spi_flash.h

@@ -28,6 +28,15 @@ extern "C" {
 
 #define SPI_FLASH_SEC_SIZE  4096    /**< SPI Flash sector size */
 
+/**
+ * @brief  Initialize SPI flash access driver
+ *
+ *  This function must be called exactly once, before any other 
+ *  spi_flash_* functions are called.
+ *
+ */
+void spi_flash_init();
+
 /**
  * @brief  Erase the Flash sector.
  *