base-files: use JSON for storing firmware validation info

base-files: use JSON for storing firmware validation info

So far firmware validation result was binary limited: it was either
successful or not. That meant various limitations, e.g.:
1) Lack of proper feedback on validation problems
2) No way of marking firmware as totally broken (impossible to install)

This change introduces JSON for storing detailed validation info. It
provides a list of performed validation tests and their results. It
allows marking firmware as non-forceable (broken image that can't be
even forced to install).
Example:
{
        "tests": {
                "fwtool_signature": true,
                "fwtool_device_match": true
        },
        "valid": true,
        "forceable": true
}

Implementation is based on *internal* check_image bash script that:
1) Uses existing validation functions
2) Provides helpers for setting extra validation info

This allows e.g. platform_check_image() to call notify_check_broken()
when needed & prevent user from bricking a device.

Right now the new JSON info is used by /sbin/sysupgrade only. It still
doesn't make use of "forceable" as that is planned for later
development.

Further plans for this feature are:
1) Expose firmware validation using some new ubus method
2) Move validation step from /sbin/sysupgrade into "sysupgrade" ubus
   method so:
   a) It's possible to safely sysupgrade using ubus only
   b) /sbin/sysupgrade can be more like just a CLI

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>