initramfs-init: disable emergency shell with norecovery_shell

An emergency shell could be used to easily extract TPM secrets from a secure boot setup. So it needs to be disabled for such setups.

There is no standard way to disable emergency shells in initramfs. initramfs-tools used by debian disables the emergency shells when the panic= parameter is set 0. With dracut the emergency shell can be disabled with rd.shell=0 1. mkinitcpio and booster don't have such option.

Ref: #25 (comment 381427)