This change is done to reduce ambiguity in some cases;
the previous intention has been to ease navigation when
staying in a particular directory, now it's been changed
in favour of convenient toplevel `git grep' in fact.

Both variants have their pros and cons, I just find myself
leaning to this one by now hence the commit.  Feel free to
provide constructive criticism :)

Some path-related bitrot has also been fixed while at that.

The variables are best influenced by targets and not directly
to avoid architecture dependent clashes.

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.