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