make: allow setting RIOT_VERSION externally

Allows for reasonable size comparisons between branches.

Also fix indentation
It is bad to indent with tabs in Makefiles, because then you cannot move blocks freely.
E.g. here `ABC=XYZ` is part of the script of `all`:
```make
all: blub.o

if $(something)
	ABC = XYZ   # ← this is a tab :)
endif
```