FS#2494 - Implement system to allow LuCI to display messages prior to flashing a new image

[openwrt-bot]

steppres:

Currently there is no system to alert users to possible issues they may encounter when flashing a new image. This has led to certain features being delayed or even withheld (the switch from swconfig -> DSA is a good example). Simply put there is no way to alert the average user to configuration breaking changes a new image might make.

A short header that contains important information could be inserted into OpenWrt images. The header is parsed and displayed via LuCI if certain criteria are/aren't met. This could be something as simple as a image version check or something more complex like configuration and package comparisons.

This is really more of a forward looking feature, but it would allow more drastic changes (changes that cannot be migrated) be made between OpenWrt versions.