nand_spl: simpc8313: move config.mk used only for nand_spl

board/sheldon/simpc8313/config.mk is used only for nand_spl.
So it should go into nand_spl/board/sheldon/simpc8313/.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

nand_spl/board/sheldon/simpc8313/Makefile

@@ -7,6 +7,8 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+include $(srctree)/$(src)/config.mk
+
 nandobj	:= $(OBJTREE)/nand_spl/
 
 LDSCRIPT= $(TOPDIR)/nand_spl/board/$(BOARDDIR)/u-boot.lds