Support semanage.mods

Aimee OS variants can now supply a `semange.mods` file in their
configuration directories.  This file contains SELinux customizations,
in the format produced by `semanage export`.  Variants can use this
mechanism to set SELinux booleans, change user/login mappings, and other
SELinux policy customization.  At build time, the base `selinux.mods`
file in the source directory is combined with the variant's file, if it
exists, and the resultant customizations are applied in a single
transaction.

Makefile

@@ -39,6 +39,11 @@ $(O)/.built: \
 	./build.sh
 	touch $(O)/.built
 
+$(O)/semanage.mods: \
+		semanage.mods \
+		$(wildcard $(CONFIGDIR)/semanage.mods)
+	cat $^ > $@
+
 $(O)/.ready: \
 		build-rootfs.sh \
 		install.packages \
@@ -46,11 +51,12 @@ $(O)/.ready: \
 		$(CONFIGDIR)/installonly.packages \
 		$(CONFIGDIR)/busybox.symlinks \
 		$(O)/linux/arch/arm64/boot/Image.gz \
+		$(O)/semanage.mods \
 		$(shell find overlay -type f) \
 		$(shell find $(CONFIGDIR)/overlay -type f 2>/dev/null) \
 		$(O)/.host-tools \
 		$(O)/.built
-	./build-rootfs.sh
+	./build-rootfs.sh "$(O)"
 	touch $(O)/.ready
 
 $(O)/squashfs.exclude: \