Tweaks to build process for embedded scripts

- Force a rebuild if a script in applets_sh is changed.

- Move the dummy usage messages for custom applets to usage.h and
  change the name from 'dummy' to 'scripted'.

- Hide an error from gen_build_files.sh if an embed directory exists
  but is empty.

- Tidy up embedded_scripts script.

v2: Remove a couple of unnecessary tests in embedded_scripts, as
    pointed out by Xabier Oneca.
    Drop the stripping of comments.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

scripts/gen_build_files.sh

@@ -23,9 +23,9 @@ custom_scripts()
 	custom_loc="$1"
 	if [ -d "$custom_loc" ]
 	then
-		for i in $(cd "$custom_loc"; ls *)
+		for i in $(cd "$custom_loc"; ls * 2>/dev/null)
 		do
-			printf "APPLET_SCRIPTED(%s, scripted, BB_DIR_USR_BIN, BB_SUID_DROP, dummy)\n" $i;
+			printf "APPLET_SCRIPTED(%s, scripted, BB_DIR_USR_BIN, BB_SUID_DROP, scripted)\n" $i;
 		done
 	fi
 }