Consolidate cross/build-rootfs.sh into 1 place

[eerhardt]

In order to build arm binaries, we currently have roughly the same script copied into the following places:

https://github.com/dotnet/coreclr/blob/master/cross/build-rootfs.sh
https://github.com/dotnet/corefx/blob/master/cross/build-rootfs.sh
https://github.com/dotnet/core-setup/blob/master/cross/build-rootfs.sh

And they are all slightly out of date with each other because someone will update coreclr for a new platform or to fix a bug, and then forget to update the other duplicates.

Also, if/when ASP.NET and the SDK need to build for arm, they will need these scripts as well.

We should consolidate this infrastructure into a single place - this repo - where the things that are common across repos should live.

/cc @weshaggard @janvorli

[janvorli]

Unifying them would also enable me to have a single crossfs and not a separate one for corefx, coreclr and core-setup.

[markwilkie]

create native build epic

[markwilkie]

It'd be good to be opportunistic here. (not blocking though)

[deinok]

@eerhardt core-rt and source-build also use the cross folder. Just to no notice.
Is this up for grabs?

[deinok]

@janvorli About the cross folder.

I'm new to arcade, but I supose that what you need is a Task (https://docs.microsoft.com/en-us/dotnet/api/microsoft.build.utilities.task?view=netframework-4.7.2) that implements all the things from the cross folder, I'm right?

[jcagme]

Don't know anything about this script but given that corefx, coreclr, core-setup and most likely aspnet are/will consuming this, I think this is a good candidate for eng/common. We could create a cross folder and copy a version there. Don't know if at this point all repos have the same version or what would determine which is to be moved but we would end up with something like eng/common/cross/build-rootfs.sh.

@weshaggard thoughts?

[weshaggard]

Yes I agree we should try and share this script under eng\common.

[jcagme]

What repo should we use to get the file from?

[deinok]

I'm not sure but I think coreclr

[janvorli]

Yes, coreclr is the right source. But it is not just the build-rootfs.sh. We need to share the whole cross folder contents except of the tryrun.cmake which is repo specific.