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>

kernel/Kconfig

@@ -450,6 +450,7 @@ config BOOT_BANNER
 config BOOT_BANNER_STRING
 	string "Boot banner string"
 	depends on BOOT_BANNER
+	default "Booting Zephyr OS build (tainted)" if TAINT
 	default "Booting Zephyr OS build"
 	help
 	  Use this option to set the boot banner.