Support arch-specific package-installs

In some cases, like linux-image-* on debian, we need to only install
packages for a specific target architecture.

Change-Id: Ic0009d0c1e121d6f3f1f21345c544e2d98f080f9

elements/package-installs/README.rst

@@ -16,6 +16,8 @@ example package-installs.yaml::
    uninstall: True
  os-collect-config:
    installtype: source
+ linux-image-amd64:
+   arch: amd64
 
 example package-installs.json::
 
@@ -33,11 +35,15 @@ the following default values::
     phase: install.d
     uninstall: False
     installtype: * (Install package for all installtypes)
+    arch: * (Install package for all architectures)
 
 Setting the installtype property causes the package only to be installed if
 the specified installtype would be used for the element. See the
 diskimage-builder docs for more information on installtypes.
 
+Setting the arch property causes the package only to be installed for the
+specified target architecture. See documentation about the ARCH variable
+for more information.
 
 DEPRECATED: Adding a file under your elements pre-install.d, install.d, or
 post-install.d directories called package-installs-<element-name> will cause

elements/package-installs/bin/package-installs-squash

@@ -46,7 +46,14 @@ def collect_data(data, filename, element_name):
         # Filter out incorrect installtypes
         installtype = params.get('installtype', None)
         elem_installtype = get_element_installtype(element_name)
-        if installtype is None or installtype == elem_installtype:
+        valid_installtype = (installtype is None or
+                             installtype == elem_installtype)
+
+        # Filter out incorrect ARCH versions
+        arch = params.get('arch', None)
+        valid_arch = arch is None or arch == os.environ['ARCH']
+
+        if valid_installtype and valid_arch:
             data[phase][install].append((pkg_name, element_name))
     return data