OMAP4 panda: fix splusb build

Signed-off-by: John Rigby <john.rigby@linaro.org>
diff --git a/include/configs/omap4_common.h b/include/configs/omap4_common.h
index ea2447f..4ba6bb1 100644
--- a/include/configs/omap4_common.h
+++ b/include/configs/omap4_common.h
@@ -183,6 +183,7 @@
 	"pxefile_addr_r=0x86000000\0"
 
 #ifdef CONFIG_SPL_USB
+#define CONFIG_BOOTCOMMAND \
 	"if usb start; then " \
 		"set autoload no; "\
 		"bootp; "\