package/nginx: fix error is nginx.service

package/nginx: fix error is nginx.service

When using systemd nginx fails to start with the error

    /usr/bin/mkdir: invalid option -- 't'

This is due to a missing space before the semicolon in nginx.service.
For systemd to handle multiple commands in ExecStartPre it requires the
semicolon to be a separate word or the use of multiple ExecStartPre.

This commit splits the multiple commands into two ExecStartPre lines
which fixes the error and improves readability.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>