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>

include/applets.src.h

@@ -22,12 +22,6 @@ s     - suid type:
         BB_SUID_REQUIRE or BB_SUID_MAYBE applet.
 */
 
-#define NOUSAGE_STR "\b"
-
-#define dummy_trivial_usage NOUSAGE_STR \
-
-#define dummy_full_usage "" \
-
 #if defined(PROTOTYPES)
 # define APPLET(name,l,s)                    int name##_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 # define APPLET_ODDNAME(name,main,l,s,help)  int main##_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;