config: fix style issues when source-ing files

Always enclose the sourced file between double-quotes (purely for
consistency; enclosing or not are both allowed by the kconfig language).

config: move backend-related options to their own file

config: hide arch and kernel selection when used as a backend

When used as a backend, it is the responsibility of the upper-layer build
system to set the target architecture and kernel.

config: hide options to debug crosstool-NG when a backend

When used as a backend, it does not make sense to debug crosstool-NG,
so hide the option.

config: hide more config options when a backend

Hide the prompts for some behavioral options, for which the upper-layer build
system is responsible for:
- parallel jobs and maximum load
- use pipes
- use custom shell

config: hide paths when used as a backend

When crosstool-NG is used as a backend, it is the responsibility of the
upper-layer build-system to properly set paths, so we just hide the
prompts in this case.

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.

scripts/populate: cleanups and misc fixes

- it's a POSIX compliant shell script: drop bash, use /bin/sh
- fix help text
- use an absolute path for sysroot
- replace "echo" with "printf"
- replace "stat -c '%i'" with "ls -1id"
- replace "pushd / popd" with "cd / cd -"
- remove superfluous break
- bail out if required lib not found, except if forced

scripts/populate: fix handling the forced libraries list-file

If a list-file is used, then each library in the file will be handled
twice (not a real issue, as the second iteration will find the library
already present, just avoid doing the job twice).

scripts/populate: properly locate the sysroot and required tools

This fixes two problems:
- the sysroot might be in a sub-directory (think SYSROOT_DIR_PREFIX)
- it is not needed to have the target tuple to properly detect the sysroot
and the required tools

As a side effect, this script is now no longer dependent on the target
tuple, and in the future, we might be able to share it across many
toolchains (when/if we can install all of them in the same place).