includes: deprecate <zephyr/zephyr.h>

Deprecate <zephyr/zephyr.h> in favor of <zephyr/kernel.h> since they are
equivalents (zephyr/zephyr.h is just a shim). Do not warn about
deprecation if already using the deprecated CONFIG_LEGACY_INCLUDE_PATH
option.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>