kbuild: add more comments for KBUILD_NOCMDDEP=1

The cmd-check for KBUILD_NOCMDDEP=1 may not be clear until you see
commit c4d5ee1398 ("kbuild: make KBUILD_NOCMDDEP=1 handle empty
built-in.o").

When a phony target (i.e. FORCE) is the only prerequisite, Kbuild
uses a tricky way to detect that the target does not exist.

Add more comments.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

scripts/Kbuild.include

@@ -170,9 +170,13 @@ cmd = @$(if $(cmd_$(1)),set -e; $($(quiet)log_print) $(delete-on-interrupt) $(cm
 ifneq ($(KBUILD_NOCMDDEP),1)
 # Check if both commands are the same including their order. Result is empty
 # string if equal. User may override this check using make KBUILD_NOCMDDEP=1
+# If the target does not exist, the *.cmd file should not be included so
+# $(savedcmd_$@) gets empty. Then, target will be built even if $(newer-prereqs)
+# happens to become empty.
 cmd-check = $(filter-out $(subst $(space),$(space_escape),$(strip $(savedcmd_$@))), \
                          $(subst $(space),$(space_escape),$(strip $(cmd_$1))))
 else
+# We still need to detect missing targets.
 cmd-check = $(if $(strip $(savedcmd_$@)),,1)
 endif
 
@@ -186,6 +190,8 @@ make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1))))
 
 # Find any prerequisites that are newer than target or that do not exist.
 # PHONY targets skipped in both cases.
+# If there is no prerequisite other than phony targets, $(newer-prereqs) becomes
+# empty even if the target does not exist. cmd-check saves this corner case.
 newer-prereqs = $(filter-out $(PHONY),$?)
 
 # It is a typical mistake to forget the FORCE prerequisite. Check it here so