efi_driver: add init function to EFI block driver

For handling added and removed block devices we need to register events
which has to be done when the driver is installed.

This patch only creates an empty init function that will be filled with
code later on. The function needs to be called before any EFI block devices
are used. Move the efi_driver_init() call to early init.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>