contrib/initramfs: add missing conf.d/zfs

When upgrading from the distribution-provided zfs-initramfs package on
root-on-zfs Ubuntu and Debian the system may fail to boot: this change
adds the missing initramfs configuration file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes openzfs#7158

contrib/initramfs/Makefile.am

@@ -1,15 +1,17 @@
 initrddir = $(datarootdir)/initramfs-tools
 
-initrd_SCRIPTS = conf-hooks.d/zfs hooks/zfs scripts/zfs scripts/local-top/zfs
+initrd_SCRIPTS = \
+	conf.d/zfs conf-hooks.d/zfs hooks/zfs scripts/zfs scripts/local-top/zfs
 
 SUBDIRS = hooks scripts
 
 EXTRA_DIST = \
+	$(top_srcdir)/contrib/initramfs/conf.d/zfs \
 	$(top_srcdir)/contrib/initramfs/conf-hooks.d/zfs \
 	$(top_srcdir)/contrib/initramfs/README.initramfs.markdown
 
 install-initrdSCRIPTS: $(EXTRA_DIST)
-	for d in conf-hooks.d hooks scripts scripts/local-top; do \
+	for d in conf.d conf-hooks.d hooks scripts scripts/local-top; do \
 	  $(MKDIR_P) $(DESTDIR)$(initrddir)/$$d; \
 	  cp $(top_srcdir)/contrib/initramfs/$$d/zfs \
 	    $(DESTDIR)$(initrddir)/$$d/; \

contrib/initramfs/conf.d/zfs

@@ -0,0 +1,8 @@
+for x in $(cat /proc/cmdline)
+do
+	case $x in
+		root=ZFS=*|root=zfs:*)
+			BOOT=zfs
+			;;
+	esac
+done