Add -p to mkdir in build-fhs-chrootenv to prevent error if directory exists

The 'init' command for an FHS chroot environment tries to create an /etc/fonts directory. This fails if it exists and the rest of the init command doesn't run. This results in the /tmp and /root directories not being created which causes failure in an FHS environment I use.

The fix here uses '-p' on 'mkdir' which prevents the script from bailing out.