download/u-boot: Re-add usage text for no-argument form

The no-argument form of the U-Boot download script prepare trees for all
boards when run with no arguments, like the corresponding script for
coreboot. The usage text for this case was removed without any changes
to the corresponding code, assume it was by mistake and add it back.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/scripts/download/u-boot

@@ -207,6 +207,9 @@ usage()
 	progname="./download u-boot"
 
 	printf "Usage:\n"
+	printf "\t%s                                # %s\n" \
+	       "${progname}" \
+	       "Download u-boot for all boards"
 	printf "\t%s [board]                        # %s\n" \
 	       "${progname}" \
 	       "Download u-boot for the given board"