samples kernel: Replace references to native_posix w native_sim

Let's replace the references to native_posix with native_sim in
the documentation, and enable this tests by default in
native_sim.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

samples/kernel/condition_variables/condvar/README.rst

@@ -20,16 +20,16 @@ the console.
 Building and Running
 ********************
 
-This application can be built and executed on Native Posix as follows:
+This application can be built and executed on :ref:`native_sim <native_sim>` as follows:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/kernel/condition_variables/condvar
    :host-os: unix
-   :board: native_posix
+   :board: native_sim
    :goals: run
    :compact:
 
-To build for another board, change "native_posix" above to that board's name.
+To build for another board, change ``native_sim`` above to that board's name.
 
 Sample Output
 =============

samples/kernel/condition_variables/condvar/sample.yaml

@@ -1,7 +1,7 @@
 tests:
   sample.kernel.cond_var:
     integration_platforms:
-      - native_posix
+      - native_sim
     tags:
       - kernel
       - condition_variables

samples/kernel/condition_variables/simple/README.rst

@@ -19,16 +19,16 @@ the console.
 Building and Running
 ********************
 
-This application can be built and executed on Native Posix as follows:
+This application can be built and executed on :ref:`native_sim <native_sim>` as follows:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/kernel/condition_variables/simple
    :host-os: unix
-   :board: native_posix
+   :board: native_sim
    :goals: run
    :compact:
 
-To build for another board, change "native_posix" above to that board's name.
+To build for another board, change ``native_sim`` above to that board's name.
 
 Sample Output
 =============

samples/kernel/condition_variables/simple/sample.yaml

@@ -1,7 +1,7 @@
 tests:
   sample.kernel.cond_var.simple:
     integration_platforms:
-      - native_posix
+      - native_sim
     tags:
       - kernel
       - condition_variables

samples/kernel/metairq_dispatch/README.rst

@@ -60,7 +60,7 @@ kHz).
 
 Note that because the test is fundamentally measuring thread
 preemption behavior, it does not run without modification on
-native_posix platforms.  In that emulation environment, threads will
+native_sim platforms. In that emulation environment, threads will
 not be preempted except at specific instrumentation points (e.g. in
 k_busy_wait()) where they will voluntarily release the CPU.