fix the compile issue for slim image (#18015)

Why I did it
The PR introduced a bug for slim image build, #17905, by which the sonic_asic_platform is missing when build docker image for slim image.

[ building ] [ target/docker-dhcp-relay.gz ]
/sonic/dockers/docker-dhcp-relay/cli-plugin-tests /sonic
/sonic
Traceback (most recent call last):
  File "/usr/local/bin/j2", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 202, in main
    output = render_command(
  File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 186, in render_command
    result = renderer.render(args.template, context)
  File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 85, in render
    return self._env \
  File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/usr/lib/python3/dist-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "/sonic/dockers/docker-dhcp-relay/Dockerfile.j2", line 48, in top-level template code
    {% if build_reduce_image_size != "y" or sonic_asic_platform != "broadcom" %}
jinja2.exceptions.UndefinedError: 'sonic_asic_platform' is undefined
make: *** [slave.mk:1072: target/docker-dhcp-relay.gz] Error 1
make: *** Waiting for unfinished jobs....
[ finished ] [ target/docker-swss-layer-bullseye.gz ]
[ finished ] [ target/docker-syncd-brcm-dnx.gz ]
make[1]: *** [Makefile.work:608: target/sonic-broadcom.bin] Error 2
make[1]: Leaving directory '/data/work/1/s'
make: *** [Makefile:41: target/sonic-broadcom.bin] Error 2
And why it slipped the PR test? PR test doesn't compile with slim option, it won't check sonic_asic_platform != "broadcom" for PR build.

Work item tracking
Microsoft ADO (number only):
How I did it
Export sonic_asic_platform for docker build in slave.mk

How to verify it
build with slim image option.

slave.mk

@@ -1069,6 +1069,7 @@ $(addprefix $(TARGET_PATH)/, $(DOCKER_IMAGES)) : $(TARGET_PATH)/%.gz : .platform
 		# Export variables for j2. Use path for unique variable names, e.g. docker_orchagent_debs
 		export include_system_eventd="$(INCLUDE_SYSTEM_EVENTD)"
 		export build_reduce_image_size="$(BUILD_REDUCE_IMAGE_SIZE)"
+		export sonic_asic_platform="$(patsubst %-$(CONFIGURED_ARCH),%,$(CONFIGURED_PLATFORM))"
 		$(eval export $(subst -,_,$(notdir $($*.gz_PATH)))_debs=$(shell printf "$(subst $(SPACE),\n,$(call expand,$($*.gz_DEPENDS),RDEPENDS))\n" | awk '!a[$$0]++'))
 		$(eval export $(subst -,_,$(notdir $($*.gz_PATH)))_pydebs=$(shell printf "$(subst $(SPACE),\n,$(call expand,$($*.gz_PYTHON_DEBS)))\n" | awk '!a[$$0]++'))
 		$(eval export $(subst -,_,$(notdir $($*.gz_PATH)))_whls=$(shell printf "$(subst $(SPACE),\n,$(call expand,$($*.gz_PYTHON_WHEELS)))\n" | awk '!a[$$0]++'))