Makefile: make clean should delete include/generated/env.in

'make sifive_unamtched_defconfig; make clean; make' fails if file
include/generated/env.in exists. 'make clean' should remove all files that
stop building.

Add file include/generated/env.in to the clean target.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>

Makefile

@@ -2187,8 +2187,8 @@ CLEAN_DIRS  += $(MODVERDIR) \
 			$(filter-out include, $(shell ls -1 $d 2>/dev/null))))
 
 CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h \
-	       drivers/video/u_boot_logo.S tools/version.h \
-	       u-boot* MLO* SPL System.map fit-dtb.blob* \
+	       include/generated/env.in drivers/video/u_boot_logo.S \
+	       tools/version.h 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 defconfig keep-syms-lto.c \