sifiveu: add 6.12 kernel as testing

Allow selecting 6.12 as testing kernel on sifiveu.

Runtime-tested:
 - HiFive Unleashed (FU540)
 - HiFive Unmatched (FU740)

Signed-off-by: Zoltan HERPAI <[email protected]>

Author: wigyori

target/linux/sifiveu/Makefile

@@ -12,6 +12,7 @@ KERNELNAME:=Image dtbs
 SUBTARGETS:=generic
 
 KERNEL_PATCHVER:=6.6
+KERNEL_TESTING_PATCHVER:=6.12
 
 include $(INCLUDE_DIR)/target.mk