build: normalize imagebuilder/sdk naming

build: normalize imagebuilder/sdk naming

We use a specific file prefix dependent on target and release except for
imagebuilder and sdk. While everything goes `openwrt-<release>-<target>`
the two go `openwrt-<sdk/imagebuilder>-<release>-<target>`.

This commit normalizes the naming:

    openwrt-x86-64-generic-squashfs-combined.img
    openwrt-x86-64-generic-squashfs-rootfs.img.gz
    openwrt-x86-64-imagebuilder.Linux-x86_64.tar.xz
    openwrt-x86-64-sdk_gcc-11.2.0_musl.Linux-x86_64.tar.xz

Previously the naming would be as following:

    openwrt-x86-64-generic-squashfs-combined.img
    openwrt-x86-64-generic-squashfs-rootfs.img.gz
    openwrt-imagebuilder-x86-64.Linux-x86_64.tar.xz
    openwrt-sdk-x86-64_gcc-11.2.0_musl.Linux-x86_64.tar.xz

Signed-off-by: Paul Spooren <[email protected]>