api: Rename legacy API files and examples

The API is not very useful these days, since it doesn't support driver
model. It is laborious to add new functions to the API as there are so
many needed.

A better approach would be to create a library containing all of U-Boot,
then have the API be generated by a script containing a list of
functions, perhaps with wildcards. Then a stub could be created, with
a list of entry points, which links with and calls through into the
library.

In preparation for heading in this direction, rename some of the
existing files and directories:

- examples/api -> examples/legacy_api
- include/api*.h -> include/legacy_api*.h
- API_BUILD to LEGACY_API_BUILD

Co-developed-by: Claude <noreply@anthropic.com>

test/lib/unicode.c

@@ -100,7 +100,7 @@ static int unicode_test_u16_strcpy(struct unit_test_state *uts)
 LIB_TEST(unicode_test_u16_strcpy, 0);
 
 /* U-Boot uses UTF-16 strings in the EFI context only. */
-#if CONFIG_IS_ENABLED(EFI_LOADER) && !defined(API_BUILD)
+#if CONFIG_IS_ENABLED(EFI_LOADER) && !defined(LEGACY_API_BUILD)
 static int unicode_test_string16(struct unit_test_state *uts)
 {
 	char buf[20];