mkarchiso: Add buildmode to export netboot artifacts

archiso/mkarchiso: Implement a buildmode to export artifacts required for netboot with IPXE. When providing the buildmode 'netboot' via profiledef.sh or the -m option, all targets necessary to create an ISO medium are built, but the components required for netboot are exported to the output dir. Optionally, it is possible to provide a set of certificates for codsigning using the -c option, where the first file is considered as the signer certificate and the second as the key.

Add _export_netboot_artifacts() to copy build artifacts to the output directory. Add _sign_netboot_artifacts() to codesign the netboot artifacts in the work directory. Add _validate_requirements_buildmode_netboot() to check for openssl. Add _build_iso_base() to implement common function calls between the 'iso' and the 'netboot' buildmodes. Add _build_buildmode_netboot() to make use of _build_iso_base(), (optionally) _sign_netboot_artifacts() and _export_netboot_artifacts(). Change _build_buildmode_iso() to make use of _build_iso_base(). Add -c as an option to mkarchiso to read in a list of file names. Unify the output of _usage() by using the same definition style for lists of strings provided to options that accept them (e.g. -c, -m, -p).

Closes #128 (closed)