kernel: boot_delay: change to busy wait instaed of wait

Intention of CONFIG_BOOT_DELAY is to delay booting of system for certain
time. Currently it is only delaying start of _main thread as delay is
created using k_sleep. This leads to putting _main thread into timeout
queue and continue kernel boot. This is causing some of undesirable
effects in some of test Automation usecase.
This patch changes k_sleep to k_busy_wait which result in delay in OS
boot instead of delaying start of _main.

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>