zephyr: Potential bugs re. 'static inline' and static variables

Prompted by some discussion on Slack, haven’t looked for problems.

If you put

static inline void f(void)
{
	static int x;

	...
}

in a header, you’ll get a separate x variable for each translation unit that includes it. It works just like a regular static function there. The inline makes no difference.

@tautologyclub might’ve found some stuff that assumes x is shared.