scripts/populate: optimise search loop

Curently, populate will iterate over all ELF (shared objects|executables)
to look for missing NEEDED DSOs, adding to the list at every iterations
of the search loop.

Instead of looking again at previously handled ELF files, recursively
resolve every ELf files.

Also, in case there are a whole lot of files (more than the shell can
accept as arguments list, or creating a command line longer than the
shell can cope with), use a temporary file with the list of files
to search for missing dependencies.

# Configuration file for sstrip tool facility

# depends on ! BACKEND

comment "sstrip"

    depends on ARCH_BINFMT_ELF

config SSTRIP

    bool

    prompt "sstrip"

    depends on ARCH_BINFMT_ELF

    help

      The sstrip utility, to maximise the striping of ELF binaries

      (executables and libraries).

      Buildroot version, forked off the original from ELFkickers.  This one

      is somewhat maintained by the buildroot guys.   

      Supports big-endian systems.