kernel/os: Allow stacks without "static"
kernel/os: Allow stacks without "static"

This commit introduces a new macro: OS_TASK_STACK_DEFINE_NOSTATIC. This
macro defines a task stack, but without a leading "static" and trailing
";".

This macro is needed to to create an array of stacks.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core