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/embedded_scripts

@@ -81,21 +81,15 @@ exec >"$target.$$"
 if [ $n -ne 0 ]
 then
 	printf '#ifdef DEFINE_SCRIPT_DATA\n'
-	if [ $n -ne 0 ]
-	then
-		printf 'const uint16_t applet_numbers[] = {\n'
-		for i in $custom_scripts $applet_scripts
-		do
-			# TODO support applets with names including invalid characters
-			printf '\tAPPLET_NO_%s,\n' $i
-		done
-		printf '};\n'
-	fi
+	printf 'const uint16_t applet_numbers[] = {\n'
+	for i in $custom_scripts $applet_scripts
+	do
+		# TODO support applets with names including invalid characters
+		printf '\tAPPLET_NO_%s,\n' $i
+	done
+	printf '};\n'
 	printf '#else\n'
-	if [ $n -ne 0 ]
-	then
-		printf 'extern const uint16_t applet_numbers[];\n'
-	fi
+	printf 'extern const uint16_t applet_numbers[];\n'
 	printf '#endif\n'
 fi