kbuild: refactor silent mode detection

Factor out $(findstring s,...).

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>

Makefile

···
 # make-4.0 (and later) keep single letter options in the 1st word of MAKEFLAGS.
 
 ifeq ($(filter 3.%,$(MAKE_VERSION)),)
-silence:=$(findstring s,$(firstword -$(MAKEFLAGS)))
+short-opts := $(firstword -$(MAKEFLAGS))
 else
-silence:=$(findstring s,$(filter-out --%,$(MAKEFLAGS)))
+short-opts := $(filter-out --%,$(MAKEFLAGS))
 endif
 
-ifeq ($(silence),s)
+ifneq ($(findstring s,$(short-opts)),)
 quiet=silent_
 KBUILD_VERBOSE = 0
 endif