Split out base squashfs.exclude

Variants no longer need to specify the full contents of the
`squashfs.exclude` file.  The "base" file in the source directory gets
combined with the one from the config directory (if it exists).

build-squashfs.sh

@@ -5,14 +5,15 @@ set -e
 
 . "${CONFIGDIR:=${PWD}}"/config
 
-IMAGESDIR="${1}"
+O="${1}"
+IMAGESDIR="${2}"
 
 mkdir -p "${IMAGESDIR}"
 mksquashfs \
     /mnt/gentoo \
     "${IMAGESDIR}"/rootfs.squashfs \
     -comp gzip \
-    -ef "${CONFIGDIR}"/squashfs.exclude \
+    -ef "${O}"/squashfs.exclude \
     -no-exports \
     -noappend \
     -wildcards