board: ti: j721s2: j721s2.env: Add explicit boot_targets

Add explicit boot_targets to indicate the specific boot sequence to
follow.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
diff --git a/board/ti/j721s2/j721s2.env b/board/ti/j721s2/j721s2.env
index 9a03b9f..a6b2255 100644
--- a/board/ti/j721s2/j721s2.env
+++ b/board/ti/j721s2/j721s2.env
@@ -13,6 +13,7 @@
 	${mtdparts}
 run_kern=booti ${loadaddr} ${rd_spec} ${fdtaddr}
 
+boot_targets=mmc1 mmc0 usb pxe dhcp
 boot=mmc
 mmcdev=1
 bootpart=1:2