Use constant expression for the model_id

Fixes gcc 6.5.0

src/emu/kernel/setup.c

@@ -1,7 +1,7 @@
 #include "kernel_priv.h"
 
 static const char model_name[] = "kernel";
-static const int model_id = 'K';
+enum { model_id = 'K' };
 
 struct model_spec model_kernel = {
 	.name = model_name,

src/emu/nanos6/setup.c

@@ -1,7 +1,7 @@
 #include "nanos6_priv.h"
 
 static const char model_name[] = "nanos6";
-static const int model_id = '6';
+enum { model_id = '6' };
 
 struct model_spec model_nanos6 = {
 	.name = model_name,

src/emu/nodes/setup.c

@@ -1,7 +1,7 @@
 #include "nodes_priv.h"
 
 static const char model_name[] = "nodes";
-static const int model_id = 'D';
+enum { model_id = 'D' };
 
 struct model_spec model_nodes = {
 	.name = model_name,

src/emu/nosv/setup.c

@@ -1,7 +1,7 @@
 #include "nosv_priv.h"
 
 static const char model_name[] = "nosv";
-static const int model_id = 'V';
+enum { model_id = 'V' };
 
 struct model_spec model_nosv = {
 	.name = model_name,