openwrt finishing touches

[Habbie]

copied from merged PR #149

This currently sits on top of #145

TODO:

• bumping curl in openwrt would be cool as we can then use websockets without spending space on verbose messages
• add websocket support to the openwrt curl package (curl: add LIBCURL_WEBSOCKETS flag openwrt/packages#25654) (openwrt package and GH workflow #149 worked around this by patching the package)
• upstream, or patch on our end, the quickjs-ng fenv stuff (https://git.musl-libc.org/cgit/musl/tree/arch/arm/bits/fenv.h). fixed: upstream already did it! bump quickjs-ng to fresh master, fixing our armv5 fenv issue #147
• figure out if, and why, we need all of TARGET_LDFLAGS += -latomic -static -lstdc++ -lgcc_eh. Especially that last one feels so wrong.
• maybe make a base image (like https://github.com/PowerDNS/base-pdns-ci-image does) to have quicker builds
• figure out actually using this for development (incremental builds of just the changed files, like you have with local development and with the armel docker debian stuff)
• expose the built binary more directly in build artifacts
• set a better (automatic) version number than 0_git
• see if we (I think the answer is yes) enforce CONFIG_LIBCURL_WEBSOCKETS=y from our build deps
• a comment says 'we can =n a lot more things' but I think there's actually not much, we should check