kernel: init: Use entropy API directly to initialize stack canary

Some sys_rand32_get() implementation will use shared state and protect
that using some synchronization primitive such as a mutex or a
semaphore.  It's too early in the boot process to use any of them,
which causes some issues.

Use the entropy API directly to set up the stack canaries.

This doesn't completely solve the problem, as some drivers will use the
same synchronization primitives anyway.  Some drivers (e.g.  the NRF5
entropy driver) provide an API to be used by ISRs that might be
suitable here, but not all drivers do that.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>