bootstd: Tidy up reporting of errors

In a few cases the error handling is not quite right. Make sure we
return the actual error in distro_efi_read_bootflow_file() rather than
-EINVAL. Return -IO when a file cannot be read. Also show the error name
if available.

This does not change operation, but does make it easier to diagnose
problems.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootmeth-uclass.c b/boot/bootmeth-uclass.c
index 2aee1e0..3b3e061 100644
--- a/boot/bootmeth-uclass.c
+++ b/boot/bootmeth-uclass.c
@@ -319,7 +319,7 @@
 		return log_msg_ret("read", ret);
 	}
 	if (size != bytes_read)
-		return log_msg_ret("bread", -EINVAL);
+		return log_msg_ret("bread", -EIO);
 	buf[size] = '\0';
 
 	*bufp = buf;