kbuild: use 'else ifeq' for checksrc to improve readability

'ifeq ... else ifeq ... endif' notation is supported by GNU Make 3.81
or later, which is the requirement for building the kernel since
commit 37d69ee30808 ("docs: bump minimal GNU Make version to 3.81").

Use it to improve the readability.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -73,14 +73,12 @@ __build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
 	@:
 
 # Linus' kernel sanity checking tool
-ifneq ($(KBUILD_CHECKSRC),0)
-  ifeq ($(KBUILD_CHECKSRC),2)
-    quiet_cmd_force_checksrc = CHECK   $<
-          cmd_force_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
-  else
-      quiet_cmd_checksrc     = CHECK   $<
-            cmd_checksrc     = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
-  endif
+ifeq ($(KBUILD_CHECKSRC),1)
+  quiet_cmd_checksrc       = CHECK   $<
+        cmd_checksrc       = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
+else ifeq ($(KBUILD_CHECKSRC),2)
+  quiet_cmd_force_checksrc = CHECK   $<
+        cmd_force_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
 endif
 
 ifneq ($(KBUILD_ENABLE_EXTRA_GCC_CHECKS),)