uml: define "generic" subtarget

uml: define "generic" subtarget

UML didn't have a subtarget defined.

While this compiled fine, the patch caused funny filenames
for the generated files to appear:
 - "openwrt-uml--vmlinux"
 - "openwrt-uml--squashfs.img"
 - "openwrt-uml--ext4.img"
 - ...

since "generic" is implied anyway, let's just set it.

Fixes: d997477775bf ("treewide: remove implicit SUBTARGET")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15713
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>