gen_build_files.sh: exclude hidden directories

gen_build_files.sh: exclude hidden directories

I am using quilt to manage patches internally. Quilt creates dir .pc
and stores unmodified files there. Since I made change in one of
Config.src quilt made a copy in .pc/xxx.patch/xxx/Config.src. When
I run make it calls gen_build_files.sh and it generates
.pc/xxx.patch/xxx/Config.in. Now when I want to pop patch quilt
thinks I have made changes to original xxx/Config.in.

IMO the best solution is just to ignore hidden directories in
gen_build_files.sh. This also results in shorter build time in case
busybox is under git/svn versioning, since it avoids searching
many directories for Config.src.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>