Fix references to the documentation files

Many boot image configuration files refer to the
appropriate documentation file, but these references
contain typos in the directory and file name. Fix
them. Also fix reference to doc/README.SPL file.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>
diff --git a/board/boundary/nitrogen6x/nitrogen6s.cfg b/board/boundary/nitrogen6x/nitrogen6s.cfg
index b5af5cc..d7d5f29 100644
--- a/board/boundary/nitrogen6x/nitrogen6s.cfg
+++ b/board/boundary/nitrogen6x/nitrogen6s.cfg
@@ -19,7 +19,7 @@
  * Foundation Inc. 51 Franklin Street Fifth Floor Boston,
  * MA 02110-1301 USA
  *
- * Refer docs/README.imxmage for more details about how-to configure
+ * Refer doc/README.imximage for more details about how-to configure
  * and create imximage boot image
  *
  * The syntax is taken as close as possible with the kwbimage