Tweaks to build process for embedded scripts

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>