Support for building and running external apps (#112)

Libraries/elf_loader/include/elf_symbol.h

@@ -0,0 +1,41 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#include "private/elf_types.h"
+#include "elf_symbol.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define ESP_ELFSYM_EXPORT(_sym)     { #_sym, (void*)&_sym }
+#define ESP_ELFSYM_END              { NULL,  NULL }
+
+/** @brief Function symbol description */
+
+struct esp_elfsym {
+    const char  *name;      /*!< Function name */
+    const void  *sym;       /*!< Function pointer */
+};
+
+/**
+ * @brief Find symbol address by name.
+ *
+ * @param sym_name - Symbol name
+ * 
+ * @return Symbol address if success or 0 if failed.
+ */
+uintptr_t elf_find_sym(const char *sym_name);
+
+#ifdef CONFIG_ELF_LOADER_CUSTOMER_SYMBOLS
+void elf_set_custom_symbols(const struct esp_elfsym* symbols);
+#endif
+
+#ifdef __cplusplus
+}
+#endif