kernel: move kernel version files to linux/generic directory

kernel: move kernel version files to linux/generic directory

To further improve cleanup and maintenance usage, move the kernel
version files to target/linux/generic directory. This permits to self
contain any change to the specific generic directory instead of having
to bload the include directory of periodic changes.

In kernel-version.mk we now use GENERIC_PLATFORM_DIR provided by
target.mk. To make this work, we need to move the inclusion of
kernel-version.mk in target.mk right after GENERIC_PLATFORM_DIR is
defined.

This also comes to permit downstream project to provide a custom generic
directory and specify the kernel version complete of the hash and the
minor version without having to affect other feeds.

In such case both generic and the target directory are provided as feeds
and OpenWrt reference these specific one instead of the generic one.

For downstream it's still suggested and preferable to all match the
shipped generic kernel minor version but this change permits to at least
enforce good practice instead of having to bloat OpenWrt include file of
all kind of downstream changes (making porting to OpenWrt mainline even
more difficult)

Link: https://github.com/openwrt/openwrt/pull/18537
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>