kernel: banner: Expose tainted builds

Due to the (potentially) hard to understand effects of blobs, it seems
prudent to make their presence more noticeable.

With this change, whenever blobs are present in the Zephyr work space,
the hello world sample output looks like this:

> *** Booting Zephyr OS build (tainted) v3.7.0-4569-gd4f8765ef20e ***
> Hello World! esp32c3_devkitm/esp32c3

Before, it looked like this:

> *** Booting Zephyr OS build v3.7.0-4568-g69c47471d187 ***
> Hello World! esp32c3_devkitm/esp32c3

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>