tools: Fix the cross-build script (sourcenetwork#460)

A cross build scripts that works for multiple target platforms and is located in `tools/scripts`.
Additionally, with this commit, the top-level `build` directory goes away.

Makefile

@@ -9,12 +9,12 @@ install:
 build:
 	go build -o build/defradb cli/defradb/main.go
 
-.PHONY: multi-build
-multi-build:
-	echo "Compiling for multiple OS and Platforms"
-	GOOS=linux GOARCH=arm go build -o build/defradb-linux-arm cli/defradb/main.go
-	GOOS=linux GOARCH=arm64 go build -o build/defradb-linux-arm64 cli/defradb/main.go
-	GOOS=freebsd GOARCH=386 go build -o build/defradb-freebsd-386 cli/defradb/main.go
+# Usage: make cross-build platforms="{platforms}"
+# platforms is specified as a comma-separated list with no whitespace, e.g. "linux/amd64,linux/arm,linux/arm64"
+# If none is specified, build for all platforms.
+.PHONY: cross-build
+cross-build:
+	bash tools/scripts/cross-build.sh $(platforms)
 
 .PHONY: start
 start: build

tools/scripts/cross-build.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+
+DEFRADB_MAIN="cli/defradb/main.go"
+BUILD_DIR="build/"
+
+platforms=$1
+if [[ -z "${platforms}" ]]; then
+    echo "Building for all platforms"
+    # A subset of the comprehensive list found at https://go.dev/doc/install/source#environment
+    platforms=(
+        "windows/amd64"
+        "windows/arm64"
+        "windows/arm"
+        "linux/amd64"
+        "linux/arm64"
+        "linux/arm"
+        "darwin/amd64"
+        "darwin/arm64"
+        # "js/wasm"
+    )
+else
+    platforms=(${platforms//,/ })
+fi
+
+for platform in "${platforms[@]}"
+do
+    platform_split=(${platform//\// })
+    GOOS="${platform_split[0]}"
+    GOARCH="${platform_split[1]}"
+    output_name=$BUILD_DIR'defradb-'$GOOS'-'$GOARCH
+    if [ "$GOOS" = "windows" ]; then
+        output_name+='.exe'
+    fi
+    if ! env GOOS="$GOOS" GOARCH="$GOARCH" go build -o $output_name $DEFRADB_MAIN; then
+        echo 'An error has occurred! Aborting the script execution...'
+        exit 1
+    fi
+    echo "Completed: ${output_name}"
+done