migrate family & bootloader_id to board.md files, add mcu family search"

[durapensa]

A cleaned-up version of #756 (supersedes that PR), including discussed migration of family & bootloader_id to board.md files per @makermelissa's suggestion.

bootloaders.json now contains only bootloader port (family) & version information, and no longer contains board information.

Tested locally such that _includes/download/board.html points to the correct downloads for CircuitPython & UF2 bootloader, and search/filter works as in #756.

[makermelissa]

At a glance this looks good, but I want to test this locally before approving.

[makermelissa]

The MCU family is showing undefined. I think the fix may be to add an if statement to the code logic, but I'm not entirely sure.

[durapensa]

I've been trying to track that down, and it should be resolved now (fix in commit above).

[makermelissa]

Thanks, the undefined option is gone for me now. However, it looks like the filters on the Blinka page have stopped functioning.

[makermelissa]

More specifically, the clicking the Filters button doesn't bring up any filters.

[durapensa]

I didn't test that! ;-) Nice catch.

Fix was: rather than add conditionals to assets/javascript/downloads.js, just make it happy by supplying it with empty mcufamily data from blinka.html.

[makermelissa]

Thanks for the quick response. It appears to be working great!