mx51evk: Do not force the rootfs type

Currently mmcrootfstype is set to ext3 type.

It is better not to force it in the env vars, because users may prefer a
different file system type, so let's get rid of 'mmcrootfstype'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
mx51evk: Change default environment to cope with OE changes

OpenEmbedded has change partitioning layout of generated image so it
does not raise warnings during the boot regarding unkown partition
being used for U-Boot.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>