board: ti: j721e: j721e.env: Add explicit boot_targets

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

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
diff --git a/board/ti/j721e/j721e.env b/board/ti/j721e/j721e.env
index 8cc8232..cb27bf5 100644
--- a/board/ti/j721e/j721e.env
+++ b/board/ti/j721e/j721e.env
@@ -31,6 +31,7 @@
 name_mcur5f0_0fw=/lib/firmware/j7200-mcu-r5f0_0-fw
 #endif
 
+boot_targets=mmc1 mmc0 usb pxe dhcp
 boot=mmc
 mmcdev=1
 bootpart=1:2