Set up build scripts for all ARM build artifacts and the firmware

- [x] Add a folder `build_scripts` where buildscripts for the various artifacts reside.
This enables re-usage of the scripts for the end-user as well as for the automated Github actions.

- [x] Add script `create_sysroot.sh`
This script pulls the *plutosdr-fw* repo with a given Tag or Commit hash from the Analog Devices Github account and alters its content according to our needs (compilation of fftw, libconfig, libfec and liquid-dsp).
The result is called *custom sysroot*. The sysroot gets zipped and uploaded to Github releases to be available end-users and the `build_binaries.sh` script (see below).

- [x] Add script `build_standalone_binaries.sh`
This script also makes use of the *custom sysroot* artifact. As it would be to much effort to re-generate the custom sysroot on every build of the binaries (especially for Github actions), the chosen version of the custom sysroot artifact is downloaded from Github releases instead.

- [x] Add script `build_firmware.sh`
This script makes use of the *custom sysroot* which was created with the script mentioned above.
Additionally, it patches and configures the Linux Kernel according to our needs and configures the Linux userspace.
Finally, it builds the firmware image called `pluto.frm`.

- [ ] Update manual.hnap.de accordingly

- No actions for the x86 binaries yet (they are already implemented as Github actions in `.github/workflows`)