Fix syntax for sh (get rid of bash-isms)

--- libbeat/scripts/Makefile.orig	2020-08-11 15:19:45 UTC
+++ libbeat/scripts/Makefile
@@ -42,7 +42,7 @@ GOFILES = $(shell find . -type f -name '*.go' 2>/dev/n
 GOFILES_NOVENDOR = $(shell find . -type f -name '*.go' -not -path "*/vendor/*" 2>/dev/null)
 GOFILES_ALL = $(GOFILES) $(shell find $(ES_BEATS) -type f -name '*.go' 2>/dev/null)
 GOPACKAGES_STRESSTESTS=$(shell find . -name '*.go' 2>/dev/null | xargs awk 'FNR>1 {nextfile} /\+build.*stresstest/ {print FILENAME; nextfile}' | xargs dirname | uniq)
-SHELL=bash
+SHELL=sh
 ES_HOST?=elasticsearch
 ES_PORT?=9200
 ES_USER?=beats
@@ -139,7 +139,7 @@ ${BEAT_NAME}.test: $(GOFILES_ALL)
 .PHONY: crosscompile
 crosscompile: ## @build Cross-compile beat for the OS'es specified in GOX_OS variable. The binaries are placed in the build/bin directory.
 crosscompile: $(GOFILES)
-ifneq ($(shell [[ $(BEAT_NAME) == journalbeat ]] && echo true ),true)
+ifneq ($(shell [ $(BEAT_NAME) = journalbeat ] && echo true ),true)
 	go ${INSTALL_CMD} github.com/mitchellh/gox
 	mkdir -p ${BUILD_DIR}/bin
 	gox -output="${BUILD_DIR}/bin/{{.Dir}}-{{.OS}}-{{.Arch}}" -os="$(strip $(GOX_OS))" -osarch="$(strip $(GOX_OSARCH))" ${GOX_FLAGS}
@@ -355,12 +355,12 @@ ifndef EXCLUDE_COMMON_UPDATE_TARGET
 update: python-env fields collect config ## @build Update expects the most recent version of libbeat in the GOPATH
 	@echo "Updating generated files for ${BEAT_NAME}"
 
-ifneq ($(shell [[ $(BEAT_NAME) == libbeat || $(BEAT_NAME) == metricbeat ]] && echo true ),true)
+ifneq ($(shell [ $(BEAT_NAME) = libbeat -o $(BEAT_NAME) = metricbeat ] && echo true ),true)
 	mkdir -p include
 	go run ${INSTALL_FLAG} ${ES_BEATS}/dev-tools/cmd/asset/asset.go -license $(LICENSE) -pkg include -in fields.yml -out include/fields.go $(BEAT_NAME)
 endif
 
-ifneq ($(shell [[ $(BEAT_NAME) == libbeat || $(BEAT_NAME) == metricbeat ]] && echo true ),true)
+ifneq ($(shell [ $(BEAT_NAME) = libbeat -o $(BEAT_NAME) = metricbeat ] && echo true ),true)
 	@# Update docs
 	@mkdir -p docs
 	@${PYTHON_ENV_EXE} ${ES_BEATS}/libbeat/scripts/generate_fields_docs.py $(PWD)/fields.yml ${BEAT_TITLE} ${ES_BEATS}