build-distro: added META_DISK_INFO support

This one can be used to override the default content
of .disk/info file (used by propagator but can find
some other uses within installer as well); the reason
being that ISO9660's Volume ID is up to 32 characters
and a file lacks that particular limitation.

features.in/build-distro/README

@@ -9,3 +9,6 @@
 META_SYSTEM_ID, META_PUBLISHER, META_PREPARER, META_APP_ID,
 META_VOL_ID, META_VOL_SET, META_BIBLIO, META_ABSTRACT;
 см. тж. genisoimagerc(5) из пакета genisoimage.
+
+При необходимости задать своё содержимое файла .disk/info,
+который также используется propagator, см. META_DISK_INFO.

features.in/build-distro/lib/90-build-distro.mk

@@ -42,7 +42,11 @@ prep: | $(GLOBAL_DEBUG) dot-disk $(WHATEVER)
 # can't use mp-showref which belongs to the metaprofile
 dot-disk:
 	@mkdir -p files/.disk
-	@echo "$(META_VOL_ID) $(DATE_F)" >files/.disk/info
+	@if [ -n "$(META_DISK_INFO)" ]; then \
+		echo "$(META_DISK_INFO)" >files/.disk/info; \
+	else \
+		echo "$(META_VOL_ID) build $(DATE_F)" >files/.disk/info; \
+	fi
 	@echo "$(ARCH)" >files/.disk/arch
 	@echo "$(DATE)" >files/.disk/date
 	@if type -t git >&/dev/null; then \