Kconfig: Rename API to LEGACY_API

Update Kconfig use use CONFIG_LEGACY_API since the API is now
deprecated.

Signed-off-by: Simon Glass <sjg@chromium.org>

legacy_api/Kconfig

@@ -1,23 +1,27 @@
-config API
-	bool "Enable U-Boot API"
+config LEGACY_API
+	bool "Enable Legacy U-Boot API"
 	depends on CC_IS_GCC
 	help
-	  This option enables the U-Boot API. See api/README for more information.
+	  This option enables the legcay U-Boot API. See legacy_api/README for
+	  more information.
 
-menu "API"
-	depends on API
+	  Note that this is deprecated and not intended to be used. It will be
+	  removed by the end of 2027.
+
+menu "LEGACY API"
+	depends on LEGACY_API
 
 config SYS_MMC_MAX_DEVICE
-	int  "Maximum number of MMC devices exposed via the API"
+	int  "Maximum number of MMC devices exposed via the legacy API"
 	default 1
 
 config EXAMPLES
-	bool "Compile API examples"
+	bool "Compile legacy API examples"
 	depends on !SANDBOX
 	default y if ARCH_QEMU_ARM
 	help
-	  U-Boot provides an API for standalone applications. Examples are
-	  provided in directory examples/.
+	  U-Boot provides an legacy API for standalone applications. Examples
+	  are provided in directory examples/.
 
 config STANDALONE_LOAD_ADDR
 	depends on EXAMPLES

legacy_api/README

@@ -24,7 +24,7 @@ U-Boot machine/arch independent API for external apps
     for example it doesn't keep states, but relies on hints from the app and
     so on
 
-  - optional (CONFIG_API)
+  - optional (CONFIG_LEGACY_API)
 
 
 2. Calls