[build] rename example platform POSIX to simulation (#4555)

Having examples/posix and src/core/posix is confusing. This commit
renames the examples posix platform to simulation.

examples/apps/cli/README.md

@@ -7,7 +7,7 @@ This example application exposes OpenThread configuration and management APIs vi
 ```bash
 $ cd <path-to-openthread>
 $ ./bootstrap
-$ make -f examples/Makefile-posix
+$ make -f examples/Makefile-simulation
 ```
 
 ## 2. Start node 1

examples/apps/cli/main.c

@@ -37,7 +37,7 @@
 
 #include "openthread-system.h"
 
-#if OPENTHREAD_EXAMPLES_POSIX
+#if OPENTHREAD_EXAMPLES_SIMULATION
 #include <setjmp.h>
 #include <unistd.h>
 
@@ -71,7 +71,7 @@ int main(int argc, char *argv[])
 {
     otInstance *instance;
 
-#if OPENTHREAD_EXAMPLES_POSIX
+#if OPENTHREAD_EXAMPLES_SIMULATION
     if (setjmp(gResetJump))
     {
         alarm(0);