binman: Support reading the offset of an ELF-file symbol

Binman needs to be able to update the contents of an ELF file after it has
been build. To support this, add a function to locate the position of a
symbol's contents within the file.

Fix the comments on bss_data.c and Symbol while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/embed_data.c b/tools/binman/test/embed_data.c
new file mode 100644
index 0000000..47d8c38
--- /dev/null
+++ b/tools/binman/test/embed_data.c
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2021 Google LLC
+ *
+ * Simple program including some embedded data that can be accessed by binman.
+ * This is used by binman tests.
+ */
+
+int first[10] = {1};
+int embed[3] __attribute__((section(".embed"))) = {0x1234, 0x5678};
+int second[10] = {1};
+
+int main(void)
+{
+	return 0;
+}