binman: Correct handling of zero bss size

Fix the check for the __bss_size symbol, since it may be 0. Unfortunately
there was no test coverage for this.

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

tools/binman/test/bss_data_zero.c

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (c) 2016 Google, Inc
+ *
+ * Simple program to create a bss_data region so the symbol can be read
+ * by binutils. This is used by binman tests.
+ */
+
+int bss_data[10];
+
+int main(void)
+{
+	bss_data[2] = 2;
+
+	return 0;
+}