Makefile: add file 'defconfig' to clean target

File 'defconfig' is a build artifact of 'make savedefconfig'.
Remove it when cleaning.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>

Makefile

@@ -2030,7 +2030,7 @@ CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h tools/version.h \
 	       boot* u-boot* MLO* SPL System.map fit-dtb.blob* \
 	       u-boot-ivt.img.log u-boot-dtb.imx.log SPL.log u-boot.imx.log \
 	       lpc32xx-* bl31.c bl31.elf bl31_*.bin image.map tispl.bin* \
-	       idbloader.img flash.bin flash.log
+	       idbloader.img flash.bin flash.log defconfig
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config include/generated spl tpl \