Fixing ownership in airootfs customization

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