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.