Fixing ownership in airootfs customization

configs/releng/build.sh: Fixing wrong ownership of files when copying overlay modifications from the config's airootfs directory to the working directory in make_customize_airootfs() by using the cp flag '--no-preserve=ownership'.

Thanks to Marcos Mello and Francois Dupoux of sysresccd (https://gitlab.com/fdupoux/sysresccd-src) for making this fix available.

Closes #11 (closed)