board: phytec: common: k3: Add missing boot source to env

We set the boot source as environment variable 'boot'.
Also include 'uart' and 'usbdfu' as possible boot sources.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>
diff --git a/board/phytec/common/k3/board.c b/board/phytec/common/k3/board.c
index 9ff861c..ebdd5fb 100644
--- a/board/phytec/common/k3/board.c
+++ b/board/phytec/common/k3/board.c
@@ -148,6 +148,12 @@
 	case BOOT_DEVICE_ETHERNET:
 		env_set("boot", "net");
 		break;
+	case BOOT_DEVICE_UART:
+		env_set("boot", "uart");
+		break;
+	case BOOT_DEVICE_DFU:
+		env_set("boot", "usbdfu");
+		break;
 	};
 
 	if (IS_ENABLED(CONFIG_PHYTEC_SOM_DETECTION_BLOCKS)) {