kernel: random: ifdef z_early_boot_rand_get

This function had a to sys_rand_get() even without random source. As
Zephyr is built with linkage garbage collection and this function is
called only if either ENTROPY_HAS_DRIVER or TEST_RANDOM_GENERATOR is
enabled and these options automatically enable a random source.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>