kernel: Implement k_sleep for Single Thread

The current z_tick_sleep return directly when building kernel for Single
Thread model. This reorganize the code to use k_busy_wait() to be time
coherent since subsystems may depend on it.

In the case of a K_FOREVER timeout is selected the Single Thread the
implementation will invoke k_cpu_idle() and the system will wait for
an interrupt saving power.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>