mkplatformfs: build rpi3 image on foundations kernel

See: https://github.com/voidlinux/void-packages/pull/12889
Closes: #150 [via git-merge-pr]

mkplatformfs.sh.in

@@ -113,7 +113,7 @@ case "$PLATFORM" in
     dockstar*) PKGS="$BASEPKG ${PLATFORM%-*}-base" ;;
     odroid-u2*) PKGS="$BASEPKG ${PLATFORM%-*}-base" ;;
     odroid-c2*) PKGS="$BASEPKG ${PLATFORM%-musl}-base" ;;
-    rpi3*) PKGS="$BASEPKG rpi3-base" ;;
+    rpi3*) PKGS="$BASEPKG rpi-base" ;;
     rpi2*) PKGS="$BASEPKG rpi-base" ;;
     rpi*) PKGS="$BASEPKG ${PLATFORM%-*}-base" ;;
     pogo*) PKGS="$BASEPKG ${PLATFORM%-*}-base" ;;