217: Simplify Target to use only to cases: BuiltIn and Custom r=Dylan-DPC a=malbarbo

In the current form, cross needs to known every target its supports.
This makes things too restrict. For example, its not possible to use a
custom target image for a target that cross does not provides a docker
image (like aarch64-unknown-linux-musl, for example). Cross refuses to
execute saying it does not support the specified target. Besides that,
to add a new docker image the cross code needs to be changed.

This changes simplifies the Target enum to have only two variants:
BuiltIn and Custom. Cross needs to known if a target is a custom target
so it can call xargo, otherwise, all BuitIn targets can be uniformly
handled.

Co-authored-by: Marco A L Barbosa <malbarbo@gmail.com>

217: Simplify Target to use only to cases: BuiltIn and Custom r=Dylan-DPC a=malbarbo

In the current form, cross needs to known every target its supports.
This makes things too restrict. For example, its not possible to use a
custom target image for a target that cross does not provides a docker
image (like aarch64-unknown-linux-musl, for example). Cross refuses to
execute saying it does not support the specified target. Besides that,
to add a new docker image the cross code needs to be changed.

This changes simplifies the Target enum to have only two variants:
BuiltIn and Custom. Cross needs to known if a target is a custom target
so it can call xargo, otherwise, all BuitIn targets can be uniformly
handled.

Co-authored-by: Marco A L Barbosa <malbarbo@gmail.com>

217: Simplify Target to use only to cases: BuiltIn and Custom r=Dylan-DPC a=malbarbo

In the current form, cross needs to known every target its supports.
This makes things too restrict. For example, its not possible to use a
custom target image for a target that cross does not provides a docker
image (like aarch64-unknown-linux-musl, for example). Cross refuses to
execute saying it does not support the specified target. Besides that,
to add a new docker image the cross code needs to be changed.

This changes simplifies the Target enum to have only two variants:
BuiltIn and Custom. Cross needs to known if a target is a custom target
so it can call xargo, otherwise, all BuitIn targets can be uniformly
handled.

Co-authored-by: Marco A L Barbosa <malbarbo@gmail.com>

217: Simplify Target to use only to cases: BuiltIn and Custom r=Dylan-DPC a=malbarbo

In the current form, cross needs to known every target its supports.
This makes things too restrict. For example, its not possible to use a
custom target image for a target that cross does not provides a docker
image (like aarch64-unknown-linux-musl, for example). Cross refuses to
execute saying it does not support the specified target. Besides that,
to add a new docker image the cross code needs to be changed.

This changes simplifies the Target enum to have only two variants:
BuiltIn and Custom. Cross needs to known if a target is a custom target
so it can call xargo, otherwise, all BuitIn targets can be uniformly
handled.

Co-authored-by: Marco A L Barbosa <malbarbo@gmail.com>

431: Allow users to pick cargo for custom target builds r=reitermarkus a=anupdhml

This is an attempt to fix #325.

We want to use the target `x86_64-alpine-linux-musl` (available from alpine's rustc) to build musl-based binaries and when using cross with something not in the standard rustc/rustup targets, cross errors out as detailed in the issue above. 

For a non-standard target, cross defaults to using `xargo` right now (behavior introduced in #217), but in our case, since we want `cargo` to run ultimately, I had to also ensure that setting `xargo = false` actually works from the cross config file. If there's a better way of handling this without breaking the current default behavior, please let me know.

Also updated the docs on xargo usage accordingly (including the parts that were lagging even without this PR).

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A working example of cross use based on the changes here are available at:
https://github.com/wayfair-tremor/tremor-runtime/blob/f72e133f971adca67f6e6b6e265a4f5d5e96380d/Cross.toml#L30-L37

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Thanks for providing a tool like cross btw! It's been really useful to us as we are looking to streamline our release process 😃 

Co-authored-by: Anup Dhamala <anupdhml+git@gmail.com>