Build xpack binaries (WIP)

[jalvz]

This is needed for the first part of central config: incorporate the enroll command in the x-pack binary.

[jalvz]

I don't feel this is the right way to do it... would it be better to exclude the whole packaging folder altogether, or to find another way to pass the right path?

[graphaelli]

I wonder if there is a way to avoid having to customize this altogether? For now, I think this needs a trailing slash?

[jalvz]

in beats the structure is:

`beats`
     `beat1`
     `beatN`
     `x-pack`
         `beat1`
         `beatN`

for us is:

`apm-server`
    `x-pack`
       `apm-server`

so we don't need the .. here: 4530fa0#diff-2a90996ecc944753a4353cab58ca5295L398

... i don't see a way around it, other than changing the structure to match the one from beats which doesn't really make sense:

`some-top-dir`
    `apm-server`
        `x-pack`
           `apm-server`

[jalvz]

@graphaelli would be great if you can have a look, particularly at the 3rd commit.
would be ok to update beats at this time of the year, or would i spoil all the ecs work?

[jalvz]

note: when the time comes, rebase and merge, not squash

[graphaelli]

A beats update should be fine. Please let me know if make update complains of duplicate fields - it shouldn't but it will mean we need to take some action if it does.

[jalvz]

redirect #1645