Add the possibility to tag linuxkit/kconfig with a custom tag instead of latest

KCONFIG_TAG variable can be used to set a custom kconfig tag.
If KCONFIG_TAG is not set, the the image is tagged as linuxkit/kconfig:latest
This is useful for projects requiring to build multiple kernels that have
different patches.
When trying to edit an unpatched kernel config after working on a patched
kernel config (same kernel version), one had to rerun make kconfig first
in order to edit the config of an unpatched kernel.
Now it is possible to generate a tegged kconfig image and then, get the wanted
config by selecting the corresponding linuxkit/kexec:tag.

Signed-off-by: Gabriel Chabot <gabriel.chabot@qarnot-computing.com>

kernel/Makefile

@@ -271,6 +271,12 @@ endif
 
 # Target for kernel config
 kconfig:
+ifeq (${KCONFIG_TAG},)
 	docker build --no-cache -f Dockerfile.kconfig \
 		--build-arg KERNEL_VERSIONS="$(KERNEL_VERSIONS)" \
 		-t linuxkit/kconfig  .
+else
+	docker build --no-cache -f Dockerfile.kconfig \
+		--build-arg KERNEL_VERSIONS="$(KERNEL_VERSIONS)" \
+		-t linuxkit/kconfig:${KCONFIG_TAG}  .
+endif