sample: smp_svr: add shell overlay

Add overlay that includes only the shell transport of mcumgr, without
bringing in BLE as well. This can be used as an alternative for the
serial transport. The `smp_svr` README was updated accordingly.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

samples/subsys/mgmt/mcumgr/smp_svr/README.rst

@@ -119,6 +119,18 @@ Zephyr. The ``smp_svr`` sample comes in different flavours.
             -- \
             -DOVERLAY_CONFIG='overlay-serial.conf;overlay-fs.conf'
 
+   .. group-tab:: Shell
+
+      To build the shell sample:
+
+      .. code-block:: console
+
+         west build \
+            -b frdm_k64f \
+            samples/subsys/mgmt/mcumgr/smp_svr \
+            -- \
+            -DOVERLAY_CONFIG='overlay-shell.conf'
+
    .. group-tab:: UDP
 
       The UDP transport for SMP supports both IPv4 and IPv6.
@@ -189,6 +201,13 @@ send a string to the remote target device and have it echo it back:
          sudo mcumgr --conntype ble --connstring ctlr_name=hci0,peer_name='Zephyr' echo hello
          hello
 
+   .. group-tab:: Shell
+
+      .. code-block:: console
+
+         mcumgr --conntype serial --connstring "/dev/ttyACM0,baud=115200" echo hello
+         hello
+
    .. group-tab:: UDP
 
       Using IPv4:

samples/subsys/mgmt/mcumgr/smp_svr/overlay-shell.conf

@@ -0,0 +1,2 @@
+# Enable the shell mcumgr transport.
+CONFIG_MCUMGR_SMP_SHELL=y