spl: unbreak CONFIG_SPL_MULTI_DTB_FIT after fixing CONFIG_OF_EMBED

With commit 9bd76b807636 ("spl: make CONFIG_OF_EMBED pass dts through
fdtgrep"), CONFIG_SPL_MULTI_DTB_FIT has been broken because
cmd_fdtgrep was now unknown in scripts/Makefile.spl after moving
it to dts/Makefile. This bug has been introduced with v2018.01.

This patch moves cmd_fdtgrep from dts/Makefile to scripts/Makefile.lib
and includes scripts/Makefile.lib in scripts/Makefile.spl.

Fixes: 9bd76b807636 ("spl: make CONFIG_OF_EMBED pass dts through fdtgrep")
Reported-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 8f19b2d..8f21653 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -522,3 +522,21 @@
 quiet_cmd_mkimage = MKIMAGE $@
 cmd_mkimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -d $< $@ \
 	$(if $(KBUILD_VERBOSE:1=), >$(MKIMAGEOUTPUT))
+
+# fdtgrep
+# ---------------------------------------------------------------------------
+# Pass the original device tree file through fdtgrep twice. The first pass
+# removes any unwanted nodes (i.e. those which don't have the
+# 'u-boot,dm-pre-reloc' property and thus are not needed by SPL. The second
+# pass removes various unused properties from the remaining nodes.
+# The output is typically a much smaller device tree file.
+ifeq ($(CONFIG_TPL_BUILD),y)
+fdtgrep_props := -b u-boot,dm-pre-reloc -b u-boot,dm-tpl
+else
+fdtgrep_props := -b u-boot,dm-pre-reloc -b u-boot,dm-spl
+endif
+quiet_cmd_fdtgrep = FDTGREP $@
+      cmd_fdtgrep = $(objtree)/tools/fdtgrep $(fdtgrep_props) -RT $< \
+		-n /chosen -n /config -O dtb | \
+	$(objtree)/tools/fdtgrep -r -O dtb - -o $@ \
+		$(addprefix -P ,$(subst $\",,$(CONFIG_OF_SPL_REMOVE_PROPS)))