nitrogen6x: README: Only calling 'make' is enough

No need to pass 'u-boot.imx' as parameter for 'make' because u-boot.imx is
built by default.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>
diff --git a/board/boundary/nitrogen6x/README b/board/boundary/nitrogen6x/README
index 5049093..6f449be 100644
--- a/board/boundary/nitrogen6x/README
+++ b/board/boundary/nitrogen6x/README
@@ -67,7 +67,7 @@
 To build U-Boot for one of the Nitrogen6x or SabreLite board:
 
 	make nitrogen6x_config
-	make u-boot.imx
+	make
 
 Note that 'nitrogen6x' is a placeholder. The complete list of supported
 board configurations is shown in tha MAINTAINERS file: