Add the possibility to tag linuxkit/kconfig with a custom tag instead of latest
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 <a6e028cedefd3a01999f61e10a841411dd0e050d@qarnot-computing.com>

JohnnyLeone/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,rn/linuxkit,deitch/linuxkit,rn/linuxkit,deitch/linuxkit,linuxkit/linuxkit,deitch/linuxkit,furious-luke/linuxkit,djs55/linuxkit,djs55/linuxkit,deitch/linuxkit,JohnnyLeone/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,rn/linuxkit,linuxkit/linuxkit,deitch/linuxkit,justincormack/linuxkit,djs55/linuxkit,djs55/linuxkit,justincormack/linuxkit,rn/linuxkit,justincormack/linuxkit,rn/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,furious-luke/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,JohnnyLeone/linuxkit,djs55/linuxkit