cmake: update desc of GENERATED_KERNEL_* variables

The previous description of the `GENERATED_KERNEL_*` cmake variables
was incorrect in that these are generated after the second link stage
when `CONFIG_USERSPACE` is enabled.

The sources in these variables are generated from `zephyr_prebuilt.elf`
for inclusion in `zephyr.elf`. However `zephyr_prebuilt.elf` is preceded
by `app_smem_unaligned_prebuilt.elf` when `CONFIG_USERSPACE` is enabled.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>