initramfs hook, should allow configuration via conf.d drop-ins

Currently the initramfs hook clears the configuration variables, before importing the user conffile.

However, most other initramfs hooks allow configuration via conf.d drop-ins. Adding such support would be useful, specifically for image/iso building. Most of the time, the installers and the bare-metal images ideally want to contain microcode for all CPUs, not the builder's CPU.