refactor(amu): unify ENABLE_AMU and ENABLE_FEAT_AMUv1

So far we have the ENABLE_AMU build option to include AMU register
handling code for enabling and context switch. There is also an
ENABLE_FEAT_AMUv1 option, solely to protect the HAFGRTR_EL2 system
register handling. The latter needs some alignment with the new feature
scheme, but it conceptually overlaps with the ENABLE_AMU option.

Since there is no real need for two separate options, unify both into a
new ENABLE_FEAT_AMU name in a first step. This is mostly just renaming at
this point, a subsequent patch will make use of the new feature handling
scheme.

Change-Id: I97d8a55bdee2ed1e1509fa9f2b09fd0bdd82736e
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/lib/extensions/amu/amu.mk b/lib/extensions/amu/amu.mk
index 0d203cb..868ab12 100644
--- a/lib/extensions/amu/amu.mk
+++ b/lib/extensions/amu/amu.mk
@@ -10,8 +10,8 @@
 			lib/extensions/amu/${ARCH}/amu_helpers.S
 
 ifneq (${ENABLE_AMU_AUXILIARY_COUNTERS},0)
-        ifeq (${ENABLE_AMU},0)
-                $(error AMU auxiliary counter support (`ENABLE_AMU_AUXILIARY_COUNTERS`) requires AMU support (`ENABLE_AMU`))
+        ifeq (${ENABLE_FEAT_AMU},0)
+                $(error AMU auxiliary counter support (`ENABLE_AMU_AUXILIARY_COUNTERS`) requires AMU support (`ENABLE_FEAT_AMU`))
         endif
 endif