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-host-tools.sh

@@ -1,12 +1,5 @@
 #!/bin/sh
 # vim: set sw=4 ts=4 sts=4 et :
 
-emerge -vnUj \
-    sys-apps/systemd \
-    sys-boot/grub \
-    sys-fs/btrfs-progs \
-    sys-fs/dosfstools \
-    sys-fs/genimage \
-    sys-fs/mtools \
-    sys-fs/squashfs-tools \
-    || exit
+< host-tools.packages xargs -ro \
+emerge -vnUj