config/config.mk: call sed in the Right Way

calling sed as sed destroys portability.
call sed as $(sed) in makefiles before the build process is started.

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="EXPERIMENTAL MIPS-64 n32 toolchain."