snap: fix and document usage of a locally build anbox image

.gitignore

@@ -3,7 +3,6 @@ parts/
 stage/
 prime/
 snap/.snapcraft
-android-images/
 *.snap
 CMakeLists.txt.user
 *.o

README.md

@@ -137,8 +137,8 @@ If you want to build the anbox snap instead you can do this with the following
 steps:
 
 ```
-$ mkdir android-images
-$ cp /path/to/android.img android-images/android.img
+$ ARCH=$(uname -m)
+$ cp /path/to/android.img data/android-images/android-$ARCH.img
 $ snapcraft
 ```
 

data/android-images/.gitignore

+android-*

data/android-images/README.md

+You can put your own android images here. They will be used
+when building the snap.
+
+You should name your images `android-$(arch).img`. For example
+`android-x86_64.img` or `android-aarch64.img`.

snap/snapcraft.yaml

@@ -95,18 +95,20 @@ parts:
     build-packages:
       - wget
     override-build: |
-      LOCAL_IMAGE=$SNAPCRAFT_PART_INSTALL/../../../android-images/android.img
+      ARCH=$(uname -m)
+      LOCAL_IMAGE="$SNAPCRAFT_PART_SRC/android-images/android-$ARCH.img"
 
-      if [ -f $LOCAL_IMAGE ]; then
+      if [ -f "$LOCAL_IMAGE" ]; then
 
         echo "Using local image $LOCAL_IMAGE"
-        cp $LOCAL_IMAGE $SNAPCRAFT_PART_INSTALL/android.img
+        cp "$LOCAL_IMAGE" "$SNAPCRAFT_PART_INSTALL"/android.img
 
       else
+        echo "android image 'data/android-images/android-$ARCH.img'" \
+             "not found, downloading one\n\n"
 
         IMAGE_PATH=
         IMAGE_NAME=
-        ARCH=$(uname -m)
 
         case "$ARCH" in
           x86_64)
@@ -115,7 +117,7 @@ parts:
             IMAGE_HASH="6b04cd33d157814deaf92dccf8a23da4dc00b05ca6ce982a03830381896a8cca"
             ;;
           *)
-            echo "ERROR: Unknown architecture $ARCH"
+            echo "ERROR: Anbox upstream doesn't provide an image for $ARCH"
             exit 1
             ;;
         esac