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: d997477775 ("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>

target/linux/uml/Makefile

@@ -12,6 +12,7 @@ ARCH:=x86_64
 BOARD:=uml
 BOARDNAME:=User Mode Linux
 FEATURES:=audio ext4 rootfs-part squashfs
+SUBTARGETS:=generic
 
 KERNEL_PATCHVER:=6.1
 KERNEL_TESTING_PATCHVER:=6.6

target/linux/uml/README.md

@@ -20,7 +20,7 @@ In your bin directory you will find a Kernel and an root file system when it's
 finished. Just run it like this:-
 
 ```shell
-./openwrt-uml-vmlinux ubd0=openwrt-uml-squashfs.img
+./openwrt-uml-generic-vmlinux ubd0=openwrt-uml-generic-squashfs.img
 ```
 
 The uml will start and eventually the serial console of the uml will be at your

target/linux/uml/generic/target.mk

@@ -0,0 +1 @@
+BOARDNAME:=Generic