kernel/idle: fix rt_thread_idle_excute in high optimization level

The rt_list_isempty has prototype of "int rt_list_isempty(const rt_list_t *l)".
So the compiler has a good reason that the rt_thread_defunct list does
not change within rt_thread_idle_excute thus optimize the "while" loop
into a "if".

So add the volatile qualifier when test against the rt_thread_defunc list.
Merge pull request #336 from grissiom/fix-idle

kernel/idle: fix rt_thread_idle_excute in high optimization level
Remove unsupport syntex for python 2.6.

Dict Comprehensions is imported in Python 2.7 & 3.0 which is PEP 274 [1]. Some
OS like CentOS 6 which is use Python 2.6 cannot support this syntex.

[1] http://legacy.python.org/dev/peps/pep-0274/

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>
Merge pull request #337 from xfguo/ignore-pep274

Remove unsupport syntex for python 2.6.
Merge branch 'master' of https://github.com/RT-Thread/rt-thread