tools: fix Makefile to clean-up fit_info and fit_check_sign

We should avoid the description in Makefile like this

ifdef CONFIG_FIT_SIGNATURE
hostprogs-y += fit_info$(SFX) fit_check_sign$(SFX)
endif

Otherwise, fit_info and fit_check_sign would never be cleaned
by "make clean".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Heiko Schocher <hs@denx.de>

tools/Makefile

@@ -60,9 +60,7 @@ hostprogs-y += mkenvimage$(SFX)
 mkenvimage$(SFX)-objs := crc32.o mkenvimage.o os_support.o
 
 hostprogs-y += dumpimage$(SFX) mkimage$(SFX)
-ifdef CONFIG_FIT_SIGNATURE
-hostprogs-y += fit_info$(SFX) fit_check_sign$(SFX)
-endif
+hostprogs-$(CONFIG_FIT_SIGNATURE) += fit_info$(SFX) fit_check_sign$(SFX)
 
 FIT_SIG_OBJS-$(CONFIG_FIT_SIGNATURE) := image-sig.o
 # Flattened device tree objects