tag/comment as part of kernel_generator and Web API

[acherm]

it's sometimes useful to tag (or even comment) some compilations...
for instance, when doing a "test" on tinyconfig: the entry in the database is not that useful or at least it should be marked as such

similarly when doing a campaign of experiments it's good to tag.

• add the argument for kernel_generator
• handle the argument and prepare the JSON for further sending it
• at the Web API level, modify the database, process the JSON, and store the information