Makefile: fix for OSX bash variable substitution

This was generating a bad config file

Signed-off-by: Michel Pollet <buserror@gmail.com>
cores; 128rfr2, tiny84 and megax8, temp fix for bad arv-libc headers

Temporary, to make the build work. Seems the latest avr-libc in debian
is rather mangled.

Signed-off-by: Michel Pollet <buserror@gmail.com>