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.

 # Dummy config file to not use a C library *at all*

 # depends on BARE_METAL

 config LIBC_none

     help

       Do not use a C library.

       This is usefull if your project is self-contained, does not rely on

       an external C library, and provides all the necessary bits.

       Most probably usefull to bootloaders, as they generally don't depend

       on the C library.

       If unsure: do *not* choose that, and use another option in the choice.