build-squashfs: Allow defining pseudo files

The `mksquashfs` command accepts a `-pf` argument, which specifies a
file that defines "pseudo files."  Each line in the file defines some
file path and an operation to perform on that path *in the generated
filesystem image*.  This allows, e.g. creating files, changing their
permissions, etc. that cannot be done in the source directory for
whatever reason.

Aimee OS now allows a variant to provide a pseudo file definition file
for `mksquashfs` by creating a `squashfs.pseudo` file in its
configuration directory.

build-squashfs.sh

@@ -14,6 +14,7 @@ mksquashfs \
     "${IMAGESDIR}"/rootfs.squashfs \
     -comp gzip \
     -ef "${O}"/squashfs.exclude \
+    -pf "${O}"/squashfs.pseudo \
     -no-exports \
     -noappend \
     -wildcards