bootstd: Rename bootdev checkers

These functions return 0 if the check passes, so the names are somewhat
confusing. Rename them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootmeth_efi_mgr.c b/boot/bootmeth_efi_mgr.c
index 2f327c1..e9d9734 100644
--- a/boot/bootmeth_efi_mgr.c
+++ b/boot/bootmeth_efi_mgr.c
@@ -36,7 +36,7 @@
 	int ret;
 
 	/* Must be an bootstd device */
-	ret = bootflow_iter_uses_system(iter);
+	ret = bootflow_iter_check_system(iter);
 	if (ret)
 		return log_msg_ret("net", ret);