binman: Support symbols in u-boot-spl-nodtb

Since this is an execuable we should be able insert symbol values into it.
Add support for this.

Use common code for this test and the original testSymbols. Use hex
consistently for the values and add some more comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 5c6663e..1dbdd0b 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -1011,6 +1011,15 @@
 entry after this one, or use a u-boot-spl entry instead (which contains
 both SPL and the device tree).
 
+SPL can access binman symbols at runtime. See:
+
+    'Access to binman entry offsets at run time (symbols)'
+
+in the binman README for more information.
+
+The ELF file 'spl/u-boot-spl' must also be available for this to work, since
+binman uses that to look up symbols to write into the SPL binary.
+
 
 
 Entry: u-boot-spl-with-ucode-ptr: U-Boot SPL with embedded microcode pointer