env_fat: use get_device_and_partition() during env save and load

Use get_device_and_partition() is better since:
1. It will call the device initialize function internally. So we can
remove the mmc intialization code to save many lines.
2. It is used by fatls/fatload/fatwrite. So saveenv & load env should
use it too.
3. It can parse the "D:P", "D", "D:", "D:auto" string to get correct
device and partition information by run-time.

Also we remove the FAT_ENV_DEVICE and FAT_ENV_PART. We use a string:
FAT_ENV_DEVICE_AND_PART.
For at91sam9m10g45ek, it is "0". That means use device 0 and if:
a)device 0 has no partition table, use the whole device as a FAT file
system.
b)device 0 has partittion table, use the partition #1.

Refer to the commit: 10a37fd7 for details of device & partition string.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>