ipqx0xx: add Generic subtarget

ipqx0xx: add Generic subtarget

Both targets miss a subtarget causing an image naming style which is
different from other all othe targets, even tho it already uses
`x/generic/` as subfolder as if the subtarget would exist.

This commit adds the Generic subtarget resulting in consistent naming.

    ~/src/openwrt/openwrt/bin/targets/ipq806x/generic$ ls
    openwrt-ipq806x-generic-netgear_d7800-initramfs-uImage
    openwrt-ipq806x-generic-netgear-d7800.manifest
    openwrt-ipq806x-generic-netgear_d7800-squashfs-factory.img
    openwrt-ipq806x-generic-netgear_d7800-squashfs-sysupgrade.bin

CC: John Crispin <john@phrozen.org>
Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 853e4dd3062df7cb5704b15d6af6730e3194b571)