BUILD: makefile: use $(cmd_MAKE) in quiet mode

It's better to see "make" entering a subdir than seeing nothing, so
let's use a command name for make. Since make 3.81, "+" needs to be
prepended in front of the command to pass the job server to the subdir.
diff --git a/include/make/verbose.mk b/include/make/verbose.mk
index 6539747..c37d513 100644
--- a/include/make/verbose.mk
+++ b/include/make/verbose.mk
@@ -12,16 +12,19 @@
 cmd_CC = $(CC)
 cmd_LD = $(LD)
 cmd_AR = $(AR)
+cmd_MAKE = +$(MAKE)
 else
 ifeq (3.81,$(firstword $(sort $(MAKE_VERSION) 3.81)))
 # 3.81 or above
 cmd_CC = $(info $   CC      $@) $(Q)$(CC)
 cmd_LD = $(info $   LD      $@) $(Q)$(LD)
 cmd_AR = $(info $   AR      $@) $(Q)$(AR)
+cmd_MAKE = $(info $   MAKE    $@) $(Q)+$(MAKE)
 else
 # 3.80 or older
 cmd_CC = $(Q)echo "  CC      $@";$(CC)
 cmd_LD = $(Q)echo "  LD      $@";$(LD)
 cmd_AR = $(Q)echo "  AR      $@";$(AR)
+cmd_MAKE = $(Q)echo "  MAKE    $@";$(MAKE)
 endif
 endif