Makefile: Skip symbolic links to files for cscope

cscope complains that it can't find files that appears to be symbolic links

cscope: cannot find file tools/binman/test/u_boot_binman_syms_bad.c
cscope: cannot find file tools/version.h

`find -L` tests properties, but name, and cscope can't cope with symbolic
links (a lot of bugs in upstream were simple closed as kinda invalid).

To work around the problem, exclude symbolic links from the cscope.files.
Note, it's done in two pass to speed up the process (`-exec realpath ...`
approach is not portable and introduces a 3x delay).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>