This feature is more generally applicable indeed;
might result in duplication due to the installer
components adding "efivars" independently but that
is to be sorted out later in those components:
- check whether it's added already sometime soon;
- maybe stop adding that at some point in the future.

install2 and rescue roots still need this too though.

EFI/UEFI is mostly about partitioning and bootloader setup,
at least from a distribution's point of view; so the
appropriate tools should be handy and firmware interface
module should not be exterminated from installer images
but get autoloaded instead.

Please note that while there exists 32-bit x86 EFI
we don't bother with it at the time being: it's relevant
to some irrelevant Xeon systems as well as for the older
Intel Macs (<2008) that are long out of fashion anyways.
That is, initially we deal with x86_64 EFI only.