buildsys: Add helper to list suid applets

buildsys: Add helper to list suid applets

Add a helper script that lists all applets that
- do or may require SUID provileges (busybox.cfg.suid)
- do not require SUID provileges (busybox.cfg.nosuid)

Some setups prefer to build two busybox binaries, one that is suid which
contains all applets that do or may require suid privileges, and a
second one for all the rest (which drops suid). To ease splitting these
two binaries, generate a list of CONFIG_ items for the suid binary.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>