[U-Boot] Add ext4 write support

[uma.shankar at samsung.com]

Hi All,
We have resolved the coding style issues and updated the copyrights correctly in the respective header files.
Ext4fs code has been made independant of ext2 code. A separate config CONFIG_CMD_EXT4 is provided for ext4ls and load,
also CONFIG_CMD_EXT4_WRITE for ext4 write feature. 

We are still keeping crc16.c file in fs/ext4 because the lib/crc16.c file ( Poly - x^16 + x^12 + x^5 + 1)
is not compatible with Linux kernel crc implementation (Poly - x^16 + x^15 + x^2 + 1). Also, fs/ubifs/crc16.c
is also giving some errors. Kernel does not mount the partition if we use this particular crc file. Hence, we are keeping
the existing crc16.c file.

We have also added a README.ext4 file in doc/ folder with steps to test the uboot ext4 feature.

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

Thanks & Regards,
Uma Shankar
Manjunatha C Achar