mx28evk: Add a target for SPI NOR boot

Introduce 'mx28evk_spi' target which will store the environment variables
into SPI NOR, which is useful when booting from SPI NOR.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>

boards.cfg

@@ -214,6 +214,7 @@ Active  arm         arm926ejs      mxs         freescale       mx23evk
 Active  arm         arm926ejs      mxs         freescale       mx28evk             mx28evk                               mx28evk:ENV_IS_IN_MMC                                                                                                             Fabio Estevam <fabio.estevam@freescale.com>
 Active  arm         arm926ejs      mxs         freescale       mx28evk             mx28evk_auart_console                 mx28evk:MXS_AUART,MXS_AUART_BASE=MXS_UARTAPP3_BASE,ENV_IS_IN_MMC                                                                  Fabio Estevam <fabio.estevam@freescale.com>
 Active  arm         arm926ejs      mxs         freescale       mx28evk             mx28evk_nand                          mx28evk:ENV_IS_IN_NAND                                                                                                            Fabio Estevam <fabio.estevam@freescale.com>
+Active  arm         arm926ejs      mxs         freescale       mx28evk             mx28evk_spi                          mx28evk:ENV_IS_IN_SPI_FLASH                                                                                                            Fabio Estevam <fabio.estevam@freescale.com>
 Active  arm         arm926ejs      mxs         olimex          mx23_olinuxino      mx23_olinuxino                        -                                                                                                                                 Marek Vasut <marek.vasut@gmail.com>
 Active  arm         arm926ejs      mxs         ppcag           bg0900              bg0900                                -                                                                                                                                 Marek Vasut <marex@denx.de>
 Active  arm         arm926ejs      mxs         sandisk         sansa_fuze_plus     sansa_fuze_plus                       -                                                                                                                                 Marek Vasut <marek.vasut@gmail.com>