image: Tidy up fit_unsupported_reset()

This function is only used in one place and does not need to use the
preprocessor. Move it to the C file and convert it to a normal function.

Drop fit_unsupported() since it is not used.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/bootm_os.c b/common/bootm_os.c
index d635037..39623f9 100644
--- a/common/bootm_os.c
+++ b/common/bootm_os.c
@@ -58,6 +58,14 @@
 }
 #endif
 
+static void __maybe_unused fit_unsupported_reset(const char *msg)
+{
+	if (CONFIG_IS_ENABLED(FIT_VERBOSE)) {
+		printf("! FIT images not supported for '%s' - must reset board to recover!\n",
+		       msg);
+	}
+}
+
 #ifdef CONFIG_BOOTM_NETBSD
 static int do_bootm_netbsd(int flag, int argc, char *const argv[],
 			   bootm_headers_t *images)