mx28evk: Introduce a new target for saving env vars to NAND

Introduce 'mx28evk_nand' target for saving environment variables into NAND.

The mx28evk board does not come with a NAND flash populated from the
factory. It comes with an empty slot (U23), which allows the insertion of a
48-pin TSOP flash device.

Tested with a K9LBG08U0D.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/boards.cfg b/boards.cfg
index 199fcfa..7725a15 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -196,7 +196,8 @@
 apx4devkit                   arm         arm926ejs   apx4devkit          bluegiga       mxs		apx4devkit
 mx23evk                      arm         arm926ejs   mx23evk             freescale      mxs		mx23evk
 m28evk                       arm         arm926ejs   m28evk              denx           mxs		m28evk
-mx28evk                      arm         arm926ejs   mx28evk             freescale      mxs		mx28evk
+mx28evk                      arm         arm926ejs   mx28evk             freescale      mxs		mx28evk:ENV_IS_IN_MMC
+mx28evk_nand                 arm         arm926ejs   mx28evk             freescale      mxs		mx28evk:ENV_IS_IN_NAND
 sc_sps_1                     arm         arm926ejs   sc_sps_1            schulercontrol mxs
 nhk8815                      arm         arm926ejs   nhk8815             st             nomadik
 nhk8815_onenand              arm         arm926ejs   nhk8815             st             nomadik       nhk8815:BOOT_ONENAND