nitrogen6x: add secure boot support

Declaring a CSF section makes the imximage tool increase the size of
data to be loaded by the BootROM and also adds a pointer to that CSF
section in the IVT header to the BootROM can check the signature.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
diff --git a/board/boundary/nitrogen6x/nitrogen6dl.cfg b/board/boundary/nitrogen6x/nitrogen6dl.cfg
index 1cdccad..5c3e961 100644
--- a/board/boundary/nitrogen6x/nitrogen6dl.cfg
+++ b/board/boundary/nitrogen6x/nitrogen6dl.cfg
@@ -20,6 +20,9 @@
 
 #define __ASSEMBLY__
 #include <config.h>
+#ifdef CONFIG_SECURE_BOOT
+CSF CONFIG_CSF_SIZE
+#endif
 #include "asm/arch/mx6-ddr.h"
 #include "asm/arch/iomux.h"
 #include "asm/arch/crm_regs.h"