[U-Boot] Add ext4 write support

[Uma Shankar]

Hi All,
We have resolved the review comments received for uboot ext4 feature. 
Ext4 code is based on existing ext2 implementation in uboot, and is capable to ls, load ext2 partitions as well.
Hence we have removed the ext2 folder from fs folder and will maintain one ext4 directory. 
Since ext4 is based on ext2, all ext2 developers name has been added to the respective file headers

To make the command line interface clean, we are maintaining cmd_ext2.c and cmd_ext4.c files. They will act as a wrapper and
a new file cmd_ext_common.c is introduced which will act as an interface file with ext4 directory.

Hence, it will avoid lot of code duplicacy and also will maintain a cleaner command interface.
Also, the current ext4 ls and load is optimized and around 10 times faster than the existing ext2 ls and load in uboot.

Thanks to Wolfgang Denk, Kim Phillips, Graeme Russ, Rob and Mike Frysinger for your valuable suggestions and review comments.

Thanks & Regards,
Uma Shankar
Manjunatha C Achar