"make package" broken for metricbeat based community beats

[fjgal]

See discuss for background information. @ruflin advised to create an issue for it

Note that is is only reproducible after applying workaround for #3294

When packaging a community beat based on metricbeat (make package) it fails with following error:

Execute /scripts/before_build.sh github.com/someuser/somebeat
install -d -m 755 /build/homedir/scripts/
install -m 755 ./vendor/github.com/elastic/beats/libbeat/scripts/migrate_beat_config_1_x_to_5_0.py /build/homedir/scripts/
Compiling import_dashboards for windows/amd64
Compiling import_dashboards for windows/386
Compiling import_dashboards for darwin/amd64
Cloning into '/go/src/github.com/tsg/gotpl'...
# disable the system/load metricset on windows
sed -i.bk 's/- load/#- load/' /build/metricbeat-win.yml
sed: can't read /build/metricbeat-win.yml: No such file or directory
make: *** [before-build] Error 2
make[1]: *** [prepare-package-cgo] Error 2
make: *** [package] Error 2

I tracked the problem down to metricbeat/Makefile (line 56-63):

# This is called by the beats packer before building starts
.PHONY: before-build
before-build:
	# disable the system/load metricset on windows
	sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.yml
	rm $(PREFIX)/metricbeat-win.yml.bk
	sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.full.yml
	rm $(PREFIX)/metricbeat-win.full.yml.bk

metricbeat/Makefile is included in the generated Makefile for the community beat, and when called it fails since the file metricbeat-win*.yml and metricbeat-win.full.yml don't exist for the community beat.

The sed commands should only be executed when packaging metricbeat and not for other beats based on it. A possible solution (probably not the cleanest) is to introduce a condition on the BEATNAME, changing the above lines to:

# This is called by the beats packer before building starts
.PHONY: before-build
before-build:
ifeq ($(BEATNAME), metricbeat)
        # disable the system/load metricset on windows
        sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.yml
        rm $(PREFIX)/metricbeat-win.yml.bk
        sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.full.yml
        rm $(PREFIX)/metricbeat-win.full.yml.bk
endif

This issue seems to be introduced by dd43dd1

I'll submit a PR with the changes described above.

Tested on 5b973a0
OS: Ubuntu 14.04
Steps to reproduce:

$ cd $GOPATH/src/github.com/someuser
$ cookiecutter $GOPATH/src/github.com/elastic/beats/generate/metricbeat/metricset
project_name [Examplebeat]: somebeat
github_name [your-github-name]: someuser
beat [somebeat]: somebeat
beat_path [github.com/someuser]: github.com/someuser
full_name [Firstname Lastname]: Some User

$ cd somebeat/
$ make setup
$ make update
$ make package