[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.

script/bootstrap

@@ -81,9 +81,9 @@ install_packages_brew()
     brew tap ArmMbed/homebrew-formulae
     brew install arm-none-eabi-gcc
 
-    # check for gcc for posix examples
+    # check for gcc for simulation
     if ! which gcc; then
-        echo 'warning: clang/gcc needed for posix examples'
+        echo 'warning: clang/gcc needed for simulation'
         echo 'warning: please install Command Line Tools from https://developer.apple.com/download/more/'
     fi
 

script/test

@@ -44,7 +44,7 @@ do_build() {
         OPTIONS="VIRTUAL_TIME_UART=1"
     fi
 
-    make -f examples/Makefile-posix ${OPTIONS}
+    make -f examples/Makefile-simulation ${OPTIONS}
 
     if [[ -n "${RADIO_DEVICE}" ]]; then
         make -f src/posix/Makefile-posix
@@ -54,7 +54,7 @@ do_build() {
     if [ "${THREAD_VERSION}" = "1.2" ]; then
         ./bootstrap
         OPTIONS="TargetTuple=${SYSTEM_TRIPLET}-1.1 THREAD_VERSION=1.1 ${OPTIONS}"
-        make -f examples/Makefile-posix ${OPTIONS}
+        make -f examples/Makefile-simulation ${OPTIONS}
 
         if [[ -n "${RADIO_DEVICE}" ]]; then
             make -f src/posix/Makefile-posix
@@ -116,7 +116,7 @@ do_package() {
     local builddir="${OT_BUILDDIR}/cmake/openthread-sim"
     (mkdir -p "${builddir}" \
         && cd "${builddir}" \
-        && cmake "${OT_SRCDIR}" -DOT_PLATFORM="posix" \
+        && cmake "${OT_SRCDIR}" -DOT_PLATFORM="simulation" \
         && make -j${OT_BUILD_JOBS} package \
         && ls "${builddir}"/openthread-simulation-*.deb)