Add assert() for debug assertions

assert() is like BUG_ON() but compiles to nothing unless DEBUG is defined.
This is useful when a condition is an error but a board reset is unlikely
to fix it, so it is better to soldier on in hope. Assertion failures should
be caught during development/test.

It turns out that assert() is defined separately in a few places in U-Boot
with various meanings. This patch cleans up some of these.

Build errors exposed by this change (and defining DEBUG) are also fixed in
this patch.

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

lib/vsprintf.c

@@ -730,3 +730,11 @@ void panic(const char *fmt, ...)
 	while (1)
 		;
 }
+
+void __assert_fail(const char *assertion, const char *file, unsigned line,
+		   const char *function)
+{
+	/* This will not return */
+	panic("%s:%u: %s: Assertion `%s' failed.", file, line, function,
+	      assertion);
+}