Add grub2-legacy for minimum list of features for grub2 compatibility

To support older versions of grub, a minimum list of features
required for boot pool for grub are added in grub2-legacy file under
zpool/compatibility.d.

The existing grub2 compatibility file lists all read-only features
that can be enabled on boot pool for grub.

Signed-off-by: Umer Saleem <usaleem@ixsystems.com>

cmd/zpool/Makefile.am

@@ -141,6 +141,7 @@ dist_zpoolcompat_DATA = \
 	%D%/compatibility.d/freebsd-11.3 \
 	%D%/compatibility.d/freenas-9.10.2 \
 	%D%/compatibility.d/grub2 \
+	%D%/compatibility.d/grub2-legacy \
 	%D%/compatibility.d/openzfs-2.0-freebsd \
 	%D%/compatibility.d/openzfs-2.0-linux \
 	%D%/compatibility.d/openzfs-2.1-freebsd \

cmd/zpool/compatibility.d/grub2-legacy

@@ -0,0 +1,8 @@
+# Minimum features which are supported by GRUB2
+async_destroy
+embedded_data
+empty_bpobj
+enabled_txg
+hole_birth
+lz4_compress
+spacemap_histogram