nitrogen6x: README: Do not refer to MAINTAINERS file

MAINTAINERS file has been removed from the project.

Replace its reference with 'boards.cfg' file instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>

board/boundary/nitrogen6x/README

@@ -70,7 +70,7 @@ To build U-Boot for one of the Nitrogen6x or SabreLite board:
 	make
 
 Note that 'nitrogen6x' is a placeholder. The complete list of supported
-board configurations is shown in tha MAINTAINERS file:
+board configurations is shown in the boards.cfg file:
 	nitrogen6q		i.MX6Q/6D	1GB
 	nitrogen6dl		i.MX6DL		1GB
 	nitrogen6s		i.MX6S		512MB