Fix build tags and add bsd back to x-compile

The build tags added in elastic#13509 didn't have an effect because only line comments can precede build tags.

This also adds back cross-compilation for freebsd and netbsd that was disabled due to Docker compilation issues. Closes elastic#13400

libbeat/scripts/Makefile

@@ -63,7 +63,7 @@ TEST_ENVIRONMENT?=false ## @testing if true, "make testsuite" runs integration t
 SYSTEM_TESTS?=false ## @testing if true, "make test" and "make testsuite" run unit tests and system tests
 STRESS_TESTS?=false ## @testing if true, "make test" and "make testsuite" run also run the stress tests
 STRESS_TEST_OPTIONS?=-timeout=20m -race -v
-GOX_OS?=linux darwin windows openbsd ## @Building List of all OS to be supported by "make crosscompile".
+GOX_OS?=linux darwin windows freebsd netbsd openbsd ## @Building List of all OS to be supported by "make crosscompile".
 GOX_OSARCH?=!darwin/arm !darwin/arm64 !darwin/386 ## @building Space separated list of GOOS/GOARCH pairs to build by "make crosscompile".
 GOX_FLAGS?= ## @building Additional flags to append to the gox command used by "make crosscompile".
 # XXX: Should be switched back to `snapshot` once the Elasticsearch

metricbeat/Makefile

@@ -6,7 +6,7 @@ TEST_ENVIRONMENT?=true
 ES_BEATS?=..
 
 # Metricbeat can only be cross-compiled on platforms not requiring CGO.
-GOX_OS=linux windows
+GOX_OS=netbsd linux windows
 GOX_FLAGS=-arch="amd64 386 arm ppc64 ppc64le"
 
 DOCS_BRANCH=$(shell grep doc-branch ../libbeat/docs/version.asciidoc | cut -c 14-)

script/generate_imports.py

@@ -27,10 +27,10 @@
 
 // Code generated by 'make imports' - DO NOT EDIT.
 
-/*
+{buildtags}/*
 {comment}
 */
-{buildtags}package {package}
+package {package}
 
 import (
 {imports}