kernel/os: Update device initialization condition

[kasjer]

Function os_dev_init() is called in two places
First possibility is when device is created in
os_dev_create() AND g_os_started is not 0
This apply to devices that are created after main
function is called (including package defined syscfg functions).

Other place that calls os_dev_init is os_dev_initialize_all() This function calls all os_dev_init() functions for requested stage. When os_dev_initalize_all() is called only devices added by bsp (often delegated to mcu) are created. Keep in mind that since g_os_started is not set yet condition is not checked.

When mcuboot starts with sysinit() enabled it calls os_dev_initalize_all() twice then executes sysinit(). Any device created in sysinit() will not be initialized since g_os_started is not set.

This change stores device init stage number and uses it when devices are initialized. For devices created by BSP initial stage is 0 so dev_init is not called during device creation but postponed till os_dev_initialize_all() is called (so despite condition change initialization sequence stays the same).

For devices created in sysinit() stage is already set to KERNEL and they are initialized during creation even though g_os_started is not checked any more. So there should not be any differences in behavior.

For MCUBoot, devices created in sysinit() will have dev_init called when created just like it would happen when OS_SCHEDULING is enabled.

Only difference is that when device initialization fails at one stage but is not marked as critical initialization will be attempted in next stage.

[sjanc]

looks OK to me but it would be good to maybe put some of information from commit message into code comment around g_os_dev_init_stage