Include date and NixOS version in systemd-boot entries

Grub configs include the NixOS version and date they were built, now
systemd can have fun too:

    version Generation 99 NixOS 17.03.1700.51a83266d1, Linux Kernel 4.9.43, Built on 2017-08-30
    version Generation 100 NixOS 17.03.1700.51a83266d1, Linux Kernel 4.9.43, Built on 2017-08-30
    version Generation 101 NixOS 17.03.1700.51a83266d1, Linux Kernel 4.9.43, Built on 2017-08-31
    version Generation 102 NixOS 17.03.1700.51a83266d1, Linux Kernel 4.9.43, Built on 2017-09-01
    version Generation 103 NixOS 17.03.1700.51a83266d1, Linux Kernel 4.9.43, Built on 2017-09-02
    version Generation 104 NixOS 17.09beta41.1b8c7786ee, Linux Kernel 4.9.46, Built on 2017-09-02
    version Generation 105 NixOS 17.09.git.1b8c778, Linux Kernel 4.9.46, Built on 2017-09-02