Get rid of the OPTIMIZE_FOR_CPU variable

This variable, together with the FIXME comment, has been added has
part of Eric Andersen's « Major buildroot facelift, step one » commit
that occured in October 2004.

Since then, no real usage has been made of OPTIMIZE_FOR_CPU, and the
initial intention has probably been lost in the memories of the
implementors.

Therefore, get rid of the variable, and just use $(ARCH) at the two
locations the variable was used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>