uclibc: proper thread default on m68k and Microblaze

uclibc: proper thread default on m68k and Microblaze

Traditionally, Buildroot has a default of enabling thread
support. However, with the current construct of the thread choice in the
uclibc package, the m68k and microblaze architecture end up with no
thread support as the default.

In order to avoid having to explicit a more complicated "default" value
for the choice, we take a simple approach: we order the 3 possible
choices by order of "preference", since Kconfig selects the first
selectable option in a choice by default.

So, NPTL is first and is the default when available. Then comes
linuxthreads which only gets selected as the default when NPTL is
available. None is offered as a last choice (in the current
implementation, it is never the default, since all architectures can
have thread support, either through NPTL or linuxthreads).

[Thomas: reworked according to Yann's comment that we could rely on the
Kconfig behavior that selects the first available choice option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>