Makefile: Minor cleanup and only set ROOTDIR if unset already.
Makefile: Minor cleanup and only set ROOTDIR if unset already.

The ROOTDIR variable is used for pretty formatting during compilation.
Some users might want to have this display the build from the top of
their tree instead of the finit build tree.

Signed-off-by: Joachim Nilsson <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/finit,westermo/finit,troglobit/finit,westermo/finit