Added 'arm' architecture support in Boards Manager

This is a necessary step to fully support ARM builds of the Arduino IDE

See arduino#3549

Author: cmaglie

arduino-core/src/cc/arduino/contributions/packages/HostDependentDownloadableContribution.java

@@ -50,7 +50,17 @@ public boolean isCompatible(Platform platform) {
     String host = getHost();
 
     if (osName.contains("Linux")) {
-      if (osArch.contains("amd64")) {
+      if (osArch.equals("arm")) {
+        // Raspberry PI, BBB or other ARM based host
+
+        // PI: "arm-linux-gnueabihf"
+        // Arch-linux on PI2: "armv7l-unknown-linux-gnueabihf"
+        // Raspbian on PI2: "arm-linux-gnueabihf"
+        // Ubuntu Mate on PI2: "arm-linux-gnueabihf"
+        // Debian 7.9 on BBB: "arm-linux-gnueabihf"
+        // Raspbian on PI Zero: "arm-linux-gnueabihf"
+        return host.matches("arm.*-linux-gnueabihf");
+      } else if (osArch.contains("amd64")) {
         return host.matches("x86_64-.*linux-gnu");
       } else {
         return host.matches("i[3456]86-.*linux-gnu");