kernel: assert if k_current_get is called pre-kernel

k_current_get is not valid pre-kernel. It will return an invalid dummy
thread or invalid memory. The invalid memory case can occur when
CURRENT_THREAD_USE_TLS is enabled.

Assert in k_current_get when called pre-kernel so offending code may be
identified.

k_is_pre_kernel is moved up in kernel.h to avoid needing a prototype for
k_is_pre_kernel.

Signed-off-by: Rob Barnes <robbarnes@google.com>