config: respect --bootloaders option again on amd64/i386

Persist LB_BOOTLOADERS in lb_binary_* configuration, since Init_config_data called from lb_binary_* calls Prepare_config which then overrides LB_BOOTLOADER_BIOS and LB_BOOTLOADER_EFI if LB_ARCHITECTURE is amd64 or i386, and these can only be restored to their proper values by the presence of LB_BOOTLOADERS.

This therefore ensures that the value of the "--bootloaders" parameter to lb_config is respected by the lb_binary_* steps.