spl: Provide more information on boot failure

If SPL fails to boot, try to provide an error code to indicate what is
wrong. For example, if a uclass is missing, this can return -EPFNOSUPPORT
(-96) which provides useful information.

Add a helper for accessing the image-loader name so we can drop the use
of #ifdefs in this code.

Put this feature behind a CONFIG_SHOW_ERRORS option to avoid increasing
the code size.

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

include/spl.h

@@ -505,6 +505,16 @@ struct spl_image_loader {
 			  struct spl_boot_device *bootdev);
 };
 
+/* Helper function for accessing the name */
+static inline const char *spl_loader_name(const struct spl_image_loader *loader)
+{
+#ifdef CONFIG_SPL_LIBCOMMON_SUPPORT
+	return loader->name;
+#else
+	return NULL;
+#endif
+}
+
 /* Declare an SPL image loader */
 #define SPL_LOAD_IMAGE(__name)					\
 	ll_entry_declare(struct spl_image_loader, __name, spl_image_loader)