Use static binaries instead of shebang scripts

With the latest version of qemu-arm-static using a shebang script with
the interpreter set to qemu-arm-static causes infinite recursion.

Fixes #2

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Dockerfile

@@ -1,4 +1,7 @@
 FROM resin/armv7hf-debian:jessie
 
 ENV QEMU_EXECVE 1
+
 COPY . /usr/bin
+
+RUN [ "qemu-arm-static", "/bin/sh", "-c", "ln -s resin-xbuild /usr/bin/cross-build-start; ln -s resin-xbuild /usr/bin/cross-build-end; ln /bin/sh /bin/sh.real" ]

cross-build-end

@@ -1,3 +0,0 @@
-#!/usr/bin/qemu-arm-static /bin/sh.real
-
-mv /bin/sh.real /bin/sh

cross-build-start

@@ -1,4 +0,0 @@
-#!/usr/bin/qemu-arm-static /bin/sh
-
-mv /bin/sh /bin/sh.real
-cp /usr/bin/sh-shim /bin/sh

resin-xbuild.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"log"
+	"os"
+	"os/exec"
+)
+
+func crossBuildStart() {
+	err := os.Remove("/bin/sh")
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = os.Link("/usr/bin/resin-xbuild", "/bin/sh")
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func crossBuildEnd() {
+	err := os.Remove("/bin/sh")
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = os.Link("/bin/sh.real", "/bin/sh")
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func runShell() {
+	cmd := exec.Command("/usr/bin/qemu-arm-static", append([]string{"-0", "/bin/sh", "/bin/sh"}, os.Args[1:]...)...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Run()
+}
+
+func main() {
+	switch os.Args[0] {
+	case "cross-build-start":
+		crossBuildStart()
+	case "cross-build-end":
+		crossBuildEnd()
+	case "/bin/sh":
+		crossBuildEnd()
+		runShell()
+		crossBuildStart()
+	}
+}

sh-shim

@@ -1,9 +0,0 @@
-#!/usr/bin/qemu-arm-static /bin/sh.real
-
-set -o errexit
-
-cp /bin/sh.real /bin/sh
-
-/bin/sh "$@"
-
-cp /usr/bin/sh-shim /bin/sh