[build]: support specifying builder mount point and workdir (#2214)

This can be used when sonic-buildimage repo is a git submodule of other
repo.

e.g) if sonic-buildimage is a git submodule of "parent" repo and
sonic-buildimage is just under the top directory of "parent" repo, we
can build sonic-buildimage like below

$ cd parent/sonic-buildimage
$ DOCKER_BUILDER_MOUNT=`realpath ../`:`realpath ../` \
  DOCKER_BUILDER_WORKDIR=`pwd` make all

Signed-off-by: Wataru Ishida <ishida@nel-america.com>

Makefile.work

@@ -63,9 +63,17 @@ OVERLAY_MODULE_CHECK := lsmod | grep "^overlay " > /dev/null 2>&1 || (echo "ERRO
 
 BUILD_TIMESTAMP := $(shell date +%Y%m%d\.%H%M%S)
 
+ifeq ($(DOCKER_BUILDER_MOUNT),)
+override DOCKER_BUILDER_MOUNT := "$(PWD):/sonic"
+endif
+
+ifeq ($(DOCKER_BUILDER_WORKDIR),)
+override DOCKER_BUILDER_WORKDIR := "/sonic"
+endif
+
 DOCKER_RUN := docker run --rm=true --privileged \
-    -v $(PWD):/sonic \
-    -w /sonic \
+    -v $(DOCKER_BUILDER_MOUNT) \
+    -w $(DOCKER_BUILDER_WORKDIR) \
     -e "http_proxy=$(http_proxy)" \
     -e "https_proxy=$(https_proxy)" \
     -i$(if $(TERM),t,)