build: Implement CONFIGDIR setting

In effort to support different builds of Aimee OS using the same
scripts, without necessarily having to fork this repository, the build
system now supports a `CONFIGDIR` setting.  When this variable is set,
files defining the target environment, such as the lists of packages to
install, the kernel configuration, the Portage configuration, etc. are
found in the path it specifes.

The reference build, for the Home Assistant Yellow board, is configured
in the `yellow` directory.  To build it, run:

```sh
CONFIGDIR=yellow ./vm-build.sh
```

build-rootfs.sh

@@ -3,7 +3,7 @@
 
 set -e
 
-. ./config
+. "${CONFIGDIR:=${PWD}}"/config
 
 mkdir -p \
     /mnt/gentoo/usr/bin \
@@ -20,16 +20,15 @@ mkdir -p /mnt/gentoo/etc/portage
 ln -snf \
     /var/db/repos/gentoo/profiles/${profile} \
     /mnt/gentoo/etc/portage/make.profile
-cp -r portage/. /mnt/gentoo/etc/portage/
 
 ROOT=/mnt/gentoo \
-PORTAGE_CONFIGROOT="${PWD}"/portage/config/target \
-< install.packages xargs -ro \
+PORTAGE_CONFIGROOT="${CONFIGDIR}"/portage/target \
+< "${CONFIGDIR}"/install.packages xargs -ro \
 ${target}-emerge -KvnuUDj --rebuilt-binaries=y
 
 ROOT=/mnt/gentoo \
-PORTAGE_CONFIGROOT="${PWD}"/portage/config/target \
-< installonly.packages xargs -ro \
+PORTAGE_CONFIGROOT="${CONFIGDIR}"/portage/target \
+< "${CONFIGDIR}"/installonly.packages xargs -ro \
 ${target}-emerge -vnuUDj
 
 ROOT=/mnt/gentoo \
@@ -51,9 +50,12 @@ while read name; do
         ln -snf busybox /mnt/gentoo/bin/"${name}" \
             || printf 'Failed to create busybox symlink for %s\n' "${name}"
     fi
-done < busybox.symlinks
+done < "${CONFIGDIR}"/busybox.symlinks
 
 rsync -rltpDO overlay/ /mnt/gentoo/
+if [ -d "${CONFIGDIR}"/overlay ]; then
+    rsync -rltpDO "${CONFIGDIR}"/overlay/ /mnt/gentoo/
+fi
 
 cp -uv /usr/${target}/usr/bin/grub-editenv /mnt/gentoo/usr/bin/