memtester: fix rebuild

Do the edit of conf-* files only when 'cc' appears at the beginning of line,
i.e. the files has not been edited. Otherwise, the 'cc' part of the cross
toolchain gets expanded, leading to the following error on 'make
memtester-rebuild':

./compile: line 3: /home/baruch/git/buildroot/output/host/usr/bin/i686-pc-linux-gnu-g/home/baruch/git/buildroot/output/host/usr/bin/i686-pc-linux-gnu-gcc: No such file or directory
Makefile:82: recipe for target 'memtester.o' failed
make[1]: *** [memtester.o] Error 127

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>