kernel: do not use k_busy_wait when on single thread
kernel: do not use k_busy_wait when on single thread

k_busy_wait() does not work when multithreading is disabled, so do not
try to wait during boot.

Fixes #14454

Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

Vudentz/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr