kernel: move z_sys_init_run_level to init.c

The function in charge of calling all init function was defined in
device.c, had a public prototype and was just used in init.c. Since this
is really an internal function tied to Kernel init code, move it to
init.c and make it static, there's no need to expose it publicly.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>