Use "make BUILD_MODULES=1 {,bzImage,zImage,vmlinux,...}" to build
your kernel - and it'll also build the modules as you go.
# add it to $(subdir-m)
both-m := $(filter $(mod-subdirs), $(subdir-y))
-SUB_DIRS := $(subdir-y)
+SUB_DIRS := $(subdir-y) $(if $(BUILD_MODULES),$(subdir-m))
MOD_SUB_DIRS := $(sort $(subdir-m) $(both-m))
ALL_SUB_DIRS := $(sort $(subdir-y) $(subdir-m) $(subdir-n) $(subdir-))
#
# Get things started.
#
-first_rule: vmlinux
+first_rule: vmlinux $(if $(BUILD_MODULES),$(obj-m))
#
# Common rules