Make variable references in Makefiles consistent

Some variable references were written as $(foo), but the majority were
written as ${foo}. This commit changes all of the variable references
to using braces.

etc/Makefile

@@ -6,15 +6,15 @@ CLEANFILES+=	rc.conf
 MK=	../mk
 include ${MK}/os.mk
 
-ifeq ($(OS),FreeBSD)
+ifeq (${OS},FreeBSD)
 SED_EXTRA=	-e 's:@TERM@:cons25:g'
 SRCS+=		rc.conf.in rc.in rc.shutdown.in
 CONF+=		devd.conf
 BIN+=		rc rc.shutdown rc.devd
-else ifeq ($(OS),Linux)
+else ifeq (${OS},Linux)
 SED_EXTRA=	-e 's:@TERM@:wsvt25:g'
 SRCS+=		rc.conf.in rc.in rc.shutdown.in
-else ifeq ($(OS),NetBSD)
+else ifeq (${OS},NetBSD)
 SED_EXTRA=	-e 's:@TERM@:wsvt25:g'
 SRCS+=		rc.conf.in rc.in rc.shutdown.in
 BIN+=		rc rc.shutdown
@@ -25,7 +25,7 @@ include ${MK}/scripts.mk
 # We can't use "ifndef" here because that treats set-but-empty
 # as not-set which is not what we want
 MKRCSYS ?= automagicplease
-ifeq ($(MKRCSYS),automagicplease)
+ifeq (${MKRCSYS},automagicplease)
 # If the user isn't picking a default, then have the
 # config go with runtime automagic detection #357247
 rc.conf: SED_EXTRA += -e '/^rc_sys=""/s:^:\#:'