Allow setting GLUON_BUILDDIR and GLUON_IMAGEDIR manually

include/gluon.mk

 ifneq ($(__gluon_inc),1)
 __gluon_inc=1
 
-GLUON_SITEDIR := $(GLUONDIR)/site
-GLUON_IMAGEDIR := $(GLUONDIR)/images
 GLUON_OPENWRTDIR := $(GLUONDIR)/openwrt
-GLUON_BUILDDIR := $(GLUONDIR)/build
+GLUON_SITEDIR := $(GLUONDIR)/site
+
+GLUON_IMAGEDIR ?= $(GLUONDIR)/images
+GLUON_BUILDDIR ?= $(GLUONDIR)/build
 
 BOARD_BUILDDIR = $(GLUON_BUILDDIR)/$(BOARD)
 BOARD_KDIR = $(BOARD_BUILDDIR)/kernel