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.

 # PPL options

 choice

     bool

     prompt "PPL version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config PPL_V_0_10_2

     bool

     prompt "0.10.2"

 endchoice

 config PPL_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.10.2" if PPL_V_0_10_2