kbuild: allow null board for spl build
Commit 33a02da0 supported "<none>" for the board field of boards.cfg.
But it missed to modify spl/Makefile.
This commit provides the flexibility so we can use "<none>" board
in SPL too.
diff --git a/spl/Makefile b/spl/Makefile
index a4d9737..55500fd 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -88,7 +88,7 @@
ifdef SOC
libs-y += $(CPUDIR)/$(SOC)/
endif
-libs-y += board/$(BOARDDIR)/
+libs-y += $(if $(BOARDDIR),board/$(BOARDDIR)/)
libs-$(HAVE_VENDOR_COMMON_LIB) += board/$(VENDOR)/common/
libs-$(CONFIG_SPL_FRAMEWORK) += common/spl/