CI: Add embedded build/test cases with buildroot, OpenWRT, etc.

[jimklimov]

Touched upon in a number of other tickets, but never made into a targeted one, I think: we have many build scenarios on NUT CI farm and plan/want many more, and "buildroot" project (often used for embedded image creation) is one worth covering too.

Related precedent:

• https://buildroot.org/
• https://github.com/buildroot/buildroot/blob/master/package/nut/nut.mk
• CI: Add a cross-build scenario #1294 - related concern generally for cross-builds
• clients/upsclient.h: include sys/types.h #2475
• CI: Add a test case for NUT with musl libc implementation (and non-fatally, a recent collectd source for good measure) #2382
• m4/nut_compiler_family.m4: fix cross-compilation #2146
• Collectd with Musl build failure since NUT version 2.8.0 #1638
• fightwarn - nutclient.{h,cpp} : fix an horde of warnings #902 (comment)
• Also Small question about building driver from source for UPS connected to OpenWRT router #2050 on OpenWRT (while at the embedded case)