ARM: imx: Fix incorrect usage of CONFIG_SYS_MMC_ENV_PART

When running the "save" command several times on a mx6qsabresd we see:

U-Boot > save
Saving Environment to MMC...
Writing to MMC(1)... done
U-Boot > save
Saving Environment to MMC...
MMC partition switch failed
U-Boot > save
Saving Environment to MMC...
Writing to MMC(1)... done
U-Boot > save
Saving Environment to MMC...
MMC partition switch failed
U-Boot > save
Saving Environment to MMC...
Writing to MMC(1)... done
U-Boot > save
Saving Environment to MMC...
MMC partition switch failed

This issue is caused by the incorrect usage of CONFIG_SYS_MMC_ENV_PART.

CONFIG_SYS_MMC_ENV_PART should be used to specify the mmc partition that stores
the environment variables.

On some imx boards it is been incorrectly used to pass the partition of kernel
and dtb files for the 'mmcpart' script variable.

Remove the CONFIG_SYS_MMC_ENV_PART usage and configure the 'mmcpart' variable
directly.

Reported-by: Jason Liu <r64343@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Jason Liu <r64343@freescale.com>