Simplify specifying packages to build/install

The Portage packages that need to be built and/or installed are now
specified in the `build.packages` and `install.packages` files,
respectively.  Similarly, packages to be installed on the host system
are specified in `host-tools.packages`.  Finally, the
`installonly.packages` file contains a list of packages that are
installed in the destination root, but not built in the sysroot
beforehand.

This allows `make` to better track when the package sets change.  It
will also make it easier to maintain different sets for different
variants in the future.

build.sh

@@ -9,13 +9,9 @@ unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
 
 ${target}-emerge -vuUDj sys-apps/util-linux
 
-${target}-emerge -vnuUDj \
-    net-misc/openssh \
-    net-misc/wget \
-    sys-apps/busybox \
-    sys-apps/systemd \
-    sys-boot/grub \
-    sys-boot/raspberrypi-firmware \
-    sys-fs/btrfs-progs \
-    --
+cat \
+    build.packages \
+    install.packages \
+    | xargs -ro \
+${target}-emerge -vnuUDj
 ${target}-emerge -v @preserved-rebuild