qemu: avoid short-form boolean deprecated warning

qemu-system-aarch64: -semihosting-config enable,target=native:
 warning: short-form boolean option 'enable' deprecated

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
diff --git a/qemu_v8.mk b/qemu_v8.mk
index 7a93fd2..a1991e8 100644
--- a/qemu_v8.mk
+++ b/qemu_v8.mk
@@ -212,7 +212,7 @@
 		-serial tcp:localhost:54320 -serial tcp:localhost:54321 \
 		-smp $(QEMU_SMP) \
 		-s -S -machine virt,secure=on -cpu cortex-a57 \
-		-d unimp -semihosting-config enable,target=native \
+		-d unimp -semihosting-config enable=on,target=native \
 		-m 1057 \
 		-bios bl1.bin \
 		-initrd rootfs.cpio.gz \