fs/cpio: accept a list of dracut.conf files

It is not unusual that projects can target multiple boards, and in
such cases, a lot is shared between boards and only the low-level
stuff differ.

Allow users to specify more than one dracut config file to cover
tose cases.

dracut does not accept more than one config file with the -c option,
but it can load more than one from a directory with --confdir. So,
we copy all the config files to a temporary directory and se that as
--confdir. However, we can't drop passing -c, or dracut would read
the system-wide /etc/dracut.conf (not sure if ${prefix} is even used),
which we do not want. So, we use an empty file as -c; /dev/null comes
in handy as an empty file.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thierry Bultel <thierry.bultel@linatsea.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

fs/cpio/Config.in

@@ -29,13 +29,15 @@ endchoice
 
 if BR2_TARGET_ROOTFS_CPIO_DRACUT
 
-config BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILE
-	string "dracut configuration file"
+config BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILES
+	string "dracut configuration files"
 	default "fs/cpio/dracut.conf"
 	help
-	  Dracut configuration file. It determines which parts of the
-	  rootfs get included in the cpio image. See dracut.conf(5) or
-	  https://github.com/dracutdevs/dracut/blob/master/man/dracut.conf.5.asc
+	  Space-separated list of Dracut configuration files. They
+	  determine which parts of the rootfs get included in the
+	  cpio image. See:
+	    dracut.conf(5)
+	    https://github.com/dracutdevs/dracut/blob/master/man/dracut.conf.5.asc
 
 	  The default configuration file is suitable for busybox init.
 	  It doesn't, however, pivot_root into a full rootfs.

fs/cpio/cpio.mk

@@ -50,9 +50,14 @@ else ifeq ($(BR2_TARGET_ROOTFS_CPIO_DRACUT),y)
 
 ROOTFS_CPIO_DEPENDENCIES += host-dracut
 
-ROOTFS_CPIO_DRACUT_CONF_FILE = $(call qstrip,$(BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILE))
-ifeq ($(BR_BUILDING):$(ROOTFS_CPIO_DRACUT_CONF_FILE),y:)
-$(error No dracut config file name specified, check your BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILE setting)
+ROOTFS_CPIO_DRACUT_CONF_FILES = $(call qstrip,$(BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILES))
+ifeq ($(BR_BUILDING),y)
+ifeq ($(ROOTFS_CPIO_DRACUT_CONF_FILES),)
+$(error No dracut config file name specified, check your BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILES setting)
+endif
+ifneq ($(words $(ROOTFS_CPIO_DRACUT_CONF_FILES)),$(words $(sort $(notdir $(ROOTFS_CPIO_DRACUT_CONF_FILES)))))
+$(error No two dracut config files can have the same basename, check your BR2_TARGET_ROOTFS_CPIO_DRACUT_CONF_FILES setting)
+endif
 endif
 
 ifeq ($(BR2_LINUX_KERNEL),y)
@@ -63,10 +68,14 @@ ROOTFS_CPIO_OPTS += --no-kernel
 endif
 
 define ROOTFS_CPIO_CMD
-	mkdir -p $(ROOTFS_CPIO_DIR)/tmp
+	mkdir -p $(ROOTFS_CPIO_DIR)/tmp $(ROOTFS_CPIO_DIR)/confdir
+	$(foreach cfg,$(ROOTFS_CPIO_DRACUT_CONF_FILES), \
+		cp $(cfg) $(ROOTFS_CPIO_DIR)/confdir/$(notdir $(cfg))
+	)
 	$(HOST_DIR)/bin/dracut \
 		$(ROOTFS_CPIO_OPTS) \
-		-c $(ROOTFS_CPIO_DRACUT_CONF_FILE) \
+		-c /dev/null \
+		--confdir $(ROOTFS_CPIO_DIR)/confdir \
 		--sysroot $(TARGET_DIR) \
 		--tmpdir $(ROOTFS_CPIO_DIR)/tmp \
 		-M \