fix(ethos-n): update build flag in tf config

The tf build configuration for the Arm(R) Ethos(TM)-N NPU driver is
using an out of date build flag and has been updated with the correct
flag.

Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>
Change-Id: Ia704c897985fb988ccadfaba7f49fa755d5a5c5c
diff --git a/tf_config/juno-enable-ethosn b/tf_config/juno-enable-ethosn
index d568e32..a75e29a 100644
--- a/tf_config/juno-enable-ethosn
+++ b/tf_config/juno-enable-ethosn
@@ -1,3 +1,3 @@
+ARM_ETHOSN_NPU_DRIVER=1
 CROSS_COMPILE=aarch64-none-elf-
-ENABLE_ETHOSN=1
 PLAT=juno