For some reason, the ModemManager build tends to freeze when built
with QEMU user emulation for arm*. Changing the build to use a single
thread only (-j1) avoids that, although the build is slower of course.
The same thing seems to happen fo oFono as well, so set that to -j1 too.
Also limit building to "armhf armv7 aarch64" since the forks are not
needed on any other architectures, to reduce build times a bit. The
other architectures can just use the upstream packages from Alpine.
25147b4aa3