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>

Makefile

@@ -853,7 +853,7 @@ quiet_cmd_split_autoconf   = SPLIT   include/autoconf.h -> include/config/*
 quiet_cmd_gen_embedded_scripts = GEN     include/embedded_scripts.h
       cmd_gen_embedded_scripts = $(srctree)/scripts/embedded_scripts include/embedded_scripts.h $(srctree)/embed $(srctree)/applets_sh
 #bbox# piggybacked generation of few .h files
-include/config/MARKER: scripts/basic/split-include include/autoconf.h $(wildcard $(srctree)/embed/*) $(srctree)/scripts/embedded_scripts
+include/config/MARKER: scripts/basic/split-include include/autoconf.h $(wildcard $(srctree)/embed/*) $(wildcard $(srctree)/applets_sh/*) $(srctree)/scripts/embedded_scripts
 	$(call cmd,split_autoconf)
 	$(call cmd,gen_bbconfigopts)
 	$(call cmd,gen_common_bufsiz)