Makefile: prefer RELEASE=y|n over RELEASE=0|1

For clarity, we now prefer y|n over 0|1 as the values of boolean options on
make command lines. This patch applies this preference to the Makefile of
the device model and tools, while RELEASE=0|1 is still supported for
backward compatibility.

Tracked-On: #5772
Signed-off-by: Junjie Mao <junjie.mao@intel.com>

devicemodel/Makefile

@@ -11,7 +11,19 @@ DM_BUILD_VERSION ?=
 DM_BUILD_TAG ?=
 
 CC ?= gcc
-RELEASE ?= 0
+
+ifndef RELEASE
+  override RELEASE := n
+else
+  # Backward-compatibility for RELEASE=(0|1)
+  ifeq ($(RELEASE),1)
+    override RELEASE := y
+  else
+    ifeq ($(RELEASE),0)
+      override RELEASE := n
+    endif
+  endif
+endif
 
 CFLAGS := -g -O0 -std=gnu11
 CFLAGS += -D_GNU_SOURCE
@@ -52,7 +64,7 @@ endif
 endif
 endif
 
-ifeq ($(RELEASE),0)
+ifeq ($(RELEASE),n)
 CFLAGS += -DDM_DEBUG
 else
 LDFLAGS += -s