doc: Update docs as per latest kernel/u-boot status.

All dependant patches are merged in U-Boot and Linux kernel now.
Update the unleashed guide to reflect that and fix the documentation
about tftp loading path as well.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>