binman: Support ELF files for U-Boot and SPL

For sandbox we want to put ELF files in the image since that is what we
need to execute. Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 4dd67d6..69b435f 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -361,6 +361,17 @@
 
 
 
+Entry: u-boot-elf: U-Boot ELF image
+-----------------------------------
+
+Properties / Entry arguments:
+    - filename: Filename of u-boot (default 'u-boot')
+
+This is the U-Boot ELF image. It does not include a device tree but can be
+relocated to any address for execution.
+
+
+
 Entry: u-boot-img: U-Boot legacy image
 --------------------------------------
 
@@ -444,6 +455,17 @@
 
 
 
+Entry: u-boot-spl-elf: U-Boot SPL ELF image
+-------------------------------------------
+
+Properties / Entry arguments:
+    - filename: Filename of SPL u-boot (default 'spl/u-boot')
+
+This is the U-Boot SPL ELF image. It does not include a device tree but can
+be relocated to any address for execution.
+
+
+
 Entry: u-boot-spl-nodtb: SPL binary without device tree appended
 ----------------------------------------------------------------