mx51evk: Update environment in order to allow booting a dt kernel

Update the environment as done in other imx boards to allow easy switching
between booting a non-dt kernel and a dt kernel.

Change CONFIG_LOADADDR to 0x92000000, so that we can have the:

- uImage at 0x92000000
- imx51-babbage.dtb at 0x91000000

,which are adequate locations in RAM to avoid overlapping.

Boot tested the following kernels:

- 2.6.35 from FSL (11.09 branch)
- 3.9-rc7 non-dt
- 3.9-rc7 dt

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
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>