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).
(transplanted from 7fec5db8933cb296c665b8526f27a779aa0c0c7a)

cc/gcc: fix building for powerpc for the 4.4.x series

Reported by: Thomas Petazzoni
http://sourceware.org/ml/crossgcc/2010-03/msg00057.html

Discussions:
http://lists.uclibc.org/pipermail/uclibc/2010-March/043697.html
http://lists.uclibc.org/pipermail/buildroot/2010-March/032790.html
http://ibot.rikers.org/%23uclibc/20100316.html.gz
(search for "copysignl", by kos_tom, blindvt and y_morin)
(transplanted from 76b1923da63d2e3b8e9010b6138e33f6a99720a8)

libc/uclibc: add uClibc 0.9.31

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
[Yann E. MORIN: mark it EXPERIMENTAL]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

cc/gcc: fix thread-less final gcc build

If threads are disabled in libc, we don't want to enable them in the
final compiler. Doing so pass the configure stage, but fails latter on
a missing <pthread.h>.

Moreover, we don't want to build libgomp if threads are disabled; its
configure script would fails anyway.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

cc/gcc: fix recent gcc build

This fix missed conversion of CT_GCC_USE_* to CT_CC_GCC_USE_*.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

complibs/gmp: fix patch against 5.0.1

config: allow libc selection in backend mode

In backend mode, allow the upper-layer build system to force the
C library selection.

config: do not force setting kernel and arch

In backend mode, only enforce the arch and/or kernel selection if the
upper-layer build system effectively forces the selection.

scripts: misc help and auto-complete fixes

- don't list samples in the main help screen
- improve the samples listing in list-samples
- don't document the 'config' action, it's long dead
- document the 'V' environment variable
- improve on START, STOP and PREFIX environment variables
- add PREFIX and V to autocomplete
- advertise auto-complete at install time

kernel/linux: add latest versions: 2.6.33.2 2.6.32.11 2.6.31.13 2.6.27.46