fix(esp_coex): move coex init from esp_system to esp_coex component

CMakev2's DEFERRED mode for idf_component_optional_requires only links
optional components already present in the dependency graph. This caused
esp_coex headers to be unavailable when building minimal examples on
targets with both WiFi and IEEE802154 (e.g. esp32c5, esp32c6).

Moving init_coexist to esp_coex removes the cross-component dependency
and ensures the init function is only compiled when esp_coex itself is
part of the build. A linker hook is added to guarantee the object file
is not silently discarded by the linker.

Made-with: Cursor

components/esp_system/startup_funcs.c

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2015-2025 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2015-2026 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -23,10 +23,6 @@
 #include "hal/uart_types.h"
 #include "hal/uart_ll.h"
 
-#if CONFIG_SW_COEXIST_ENABLE || CONFIG_EXTERNAL_COEX_ENABLE
-#include "private/esp_coexist_internal.h"
-#endif
-
 #if CONFIG_PM_ENABLE || CONFIG_PM_WORKAROUND_FREQ_LIMIT_ENABLED
 #include "esp_pm.h"
 #include "esp_private/pm_impl.h"
@@ -127,15 +123,6 @@ ESP_SYSTEM_INIT_FN(init_apb_dma, SECONDARY, BIT(0), 203)
 }
 #endif
 
-#if CONFIG_SW_COEXIST_ENABLE || CONFIG_EXTERNAL_COEX_ENABLE
-ESP_SYSTEM_INIT_FN(init_coexist, SECONDARY, BIT(0), 204)
-{
-    esp_coex_adapter_register(&g_coex_adapter_funcs);
-    coex_pre_init();
-    return ESP_OK;
-}
-#endif // CONFIG_SW_COEXIST_ENABLE || CONFIG_EXTERNAL_COEX_ENABLE
-
 #if SOC_RECOVERY_BOOTLOADER_SUPPORTED
 ESP_SYSTEM_INIT_FN(init_bootloader_offset, SECONDARY, BIT(0), 205)
 {