lib: time: hook uthread_schedule() into udelay()

lib: time: hook uthread_schedule() into udelay()

Introduce a uthread scheduling loop into udelay() when CONFIG_UTHREAD
is enabled. This means that any uthread calling into udelay() may yield
to uthread and be scheduled again later. There is no delay in the
scheduling loop because tests have shown that such a delay can have a
detrimental effect on the console (input drops characters).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>