The l4t-unify.sh script is used to pull together the multiple repos
NVIDIA hosts for Jetson Linux (pre-R36) into a single tree, such as the
one at https://github.com/OE4T/linux-tegra-4.9 .
Start by using the source_sync.sh in the L4T kit to populate the
various source trees from NVIDIA's repos. Then copy the scripts here
into the Linux_for_Tegra directory.
Make sure the synced repos are on the right branch by using find
with the docheckout.sh script:
$ find sources -type d -name .git -exec $PWD/docheckout.sh {} \;
Next, initialize a git workspace for your unified kernel tree, and run
the l4t-unify.sh script to populate the subtrees in your new workspace
from the contents you downloaded above.
You will then need to apply a patch like this one to rework the modifications NVIDIA made to the kernel build system for the unified layout.
If you use the Yocto Project's kernel tools, you'll also need a modification to support the "overlay" approach NVIDIA took for its drivers. See this recipe.
Use at your own risk. The script is not likely to be reusable, even across L4T versions, without modification.