imx: mx6ullevk: support plugin

Add plugin code for mx6ullevk.
Define CONFIG_USE_IMXIMG_PLUGIN in defconfig file to use plugin code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/board/freescale/mx6ullevk/imximage.cfg b/board/freescale/mx6ullevk/imximage.cfg
index 4604b62..1a21b49 100644
--- a/board/freescale/mx6ullevk/imximage.cfg
+++ b/board/freescale/mx6ullevk/imximage.cfg
@@ -29,7 +29,7 @@
 BOOT_FROM	sd
 #endif
 
-#ifdef CONFIG_USE_PLUGIN
+#ifdef CONFIG_USE_IMXIMG_PLUGIN
 /*PLUGIN    plugin-binary-file    IRAM_FREE_START_ADDR*/
 PLUGIN	board/freescale/mx6ullevk/plugin.bin 0x00907000
 #else