multilib: Ensure we map the SYSTEMD_PACKAGES variable

multilib: Ensure we map the SYSTEMD_PACKAGES variable

If we don't do this, systemd.bbclase will complain to unable to find multilib
packages since PACKAGES is expand with mlprefix, but SYSTEMD_PACKAGES is not,
like in ntp.inc:

    $grep PACKAGES meta-oe/meta-networking/recipes-support/ntp/ntp.inc
    PACKAGES += "ntpdate sntp ${PN}-tickadj ${PN}-utils"
    SYSTEMD_PACKAGES = "${PN} ntpdate sntp"
    $

    $bitbake ntp
    ERROR: ntpdate does not appear in package list, please add it
    ERROR: sntp does not appear in package list, please add it
    $

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>