cmd_elf: Fix broken bootvx command

Fix broken bootvx command.

Signed-off-by: Reinhard Arlt <reinhard.arlt@esd.eu>
diff --git a/common/cmd_elf.c b/common/cmd_elf.c
index a667a46..ab9c7e3 100644
--- a/common/cmd_elf.c
+++ b/common/cmd_elf.c
@@ -198,7 +198,7 @@
 	 * defaults to 0x4200
 	 */
 	tmp = getenv("bootaddr");
-	if (tmp)
+	if (!tmp)
 		bootaddr = CONFIG_SYS_VXWORKS_BOOT_ADDR;
 	else
 		bootaddr = simple_strtoul(tmp, NULL, 16);