Use overlays in buildroot configurations

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Support ramfs from buildroot

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Add support for uClibc test suite

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Copy rootfs.pio to output directory

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>