build: optionally disable target name in filenames

build: optionally disable target name in filenames

The target/subtarget is included in all image filenames by default. This
is useful if the same profile exists in multiple targets or
developers/users want an obvious related of images to targets.

However this information can also be confusing at it adds a lengthy
string to sysupgrade images containing an information which is likely
unknown to regular users: The target/subtarget.

This commit therefore adds the new image option called
CONFIG_TARGET_FILENAMES to make the target/subtarget filename part
optional. It is ENABLED by default. To archive this the target/subtarget
is added if CONFIG_VERSIONOPT is disabled  and CONFIG_TARGET_FILENAMES
default to `y`. This keeps the current behaviour for OpenWrt &
downstream projects while making the target/subtarget inclusion still
optional.

Newly produced files would with enabled CONFIG_VERSIONOPT and disabled
CONFIG_TARGET_FILENAMES look like the following:

* openwrt-linksys_wrt3200acm-initramfs-kernel.bin
* openwrt-linksys_wrt3200acm.manifest
* openwrt-linksys_wrt3200acm-squashfs-factory.img
* openwrt-linksys_wrt3200acm-squashfs-sysupgrade.bin

ImageBuilder and SDK archive file names are generated individually and
not influenced by the option.

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