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-rootfs.sh

@@ -24,19 +24,13 @@ cp -r portage/. /mnt/gentoo/etc/portage/
 
 ROOT=/mnt/gentoo \
 PORTAGE_CONFIGROOT=/mnt/gentoo \
-${target}-emerge -Kvnj \
-    sys-apps/busybox \
-    sys-apps/systemd \
-    net-misc/openssh \
-    net-misc/wget \
-    sys-fs/btrfs-progs \
-    --
+< install.packages xargs -ro \
+${target}-emerge -Kvnj
+
 ROOT=/mnt/gentoo \
 PORTAGE_CONFIGROOT=/mnt/gentoo \
-${target}-emerge -vnj \
-    net-wireless/wireless-regdb \
-    sys-kernel/linux-firmware \
-    --
+< installonly.packages xargs -ro \
+${target}-emerge -vnj
 
 ROOT=/mnt/gentoo \
 locale-gen