Makefile: add a rule to only install libfdt

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

---
Patch not sent upstream.

It's really specific to buildroot, and is probably not
good (aka generic) enough to be pushed upstream.

diff --git a/Makefile b/Makefile
index 1169e6c..39e7190 100644
--- a/Makefile
+++ b/Makefile
@@ -160,10 +160,12 @@ endif
 # intermediate target and building them again "for real"
 .SECONDARY: $(DTC_GEN_SRCS) $(CONVERT_GEN_SRCS)
 
-install: all $(SCRIPTS)
+install: all $(SCRIPTS) libfdt_install
 	@$(VECHO) INSTALL
 	$(INSTALL) -d $(DESTDIR)$(BINDIR)
 	$(INSTALL) $(BIN) $(SCRIPTS) $(DESTDIR)$(BINDIR)
+
+libfdt_install: libfdt
 	$(INSTALL) -d $(DESTDIR)$(LIBDIR)
 	$(INSTALL) $(LIBFDT_lib) $(DESTDIR)$(LIBDIR)
 	ln -sf $(notdir $(LIBFDT_lib)) $(DESTDIR)$(LIBDIR)/$(LIBFDT_soname)