linux: generate refs automatically

Generate a Kconfig choice/endchoice menu with a list of the latest Git
references for Linus, Stable, Next and Development trees.

The patch introduces Git References that operates in 2 modes: Default
and User. Depending on the mode and tree, the Git References behaves
as follows:

* Default Mode: Linus, Next and Stable trees: latest-and-greatest +
static refs.
* Default Mode: Development trees: static refs (included in this patch).
* User Mode: Linux, Next, Stable and Development trees: latest 15 Git
References found upstream + static refs.

The script included (generate_refs.py) will create the Kconfig choice
options with the latest and greatest kernel release references provided
by kernel.org/releases.json including the static references defined
in the YAML files. Every time make is run, the script will check for
the date of the last modification, and will force an update if more
than 24h has passed since last modification. The generated files are:
refs/default/Kconfig.{linus,next,stable}.

refs/default/Kconfig.{btrfs-devel,cel-linux,jlayton-linux,kdevops-linus,
mcgrof-linus,mcgrof-next} files are autogenerated using their respective
YAML static configuration files. This is to be able to run gitrefs
subcommand and automatically generate the latest references including
the static ones (refs/user/Kconfig.*). This User Mode will be used until
refs/user/Kconfig.* files are cleared.

Signed-off-by: Daniel Gomez <[email protected]>
Reviewed-by: Luis Chamberlain <[email protected]>
Signed-off-by: Luis Chamberlain <[email protected]>

Author: dkruces

kconfig.Makefile

@@ -37,7 +37,7 @@ $(KCONFIG_DIR)/mconf:
 	$(MAKE) -C $(KCONFIG_DIR) mconf
 
 PHONY += menuconfig
-menuconfig: $(KCONFIG_DIR)/mconf include/config/project.release Kconfig
+menuconfig: $(KCONFIG_DIR)/mconf include/config/project.release Kconfig _refs-default
 	@$< Kconfig
 
 $(KCONFIG_DIR)/nconf: