[U-Boot] Add ext4 write support

[uma.shankar at samsung.com]

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.

We have also added a README file in fs/ext4/ 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