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>