target: add support to kill/restart and ADB connection

Sometimes it could be useful to disconnect from the target, for example
when a "pass thought" energy monitor is used to control the USB
connection to the device.

This patch adds a couple of utility methods which allows to kill the ADB
server and restart it while waiting for the device to be ready to accept
new connections.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
diff --git a/devlib/target.py b/devlib/target.py
index 8a38a8c..e9b24df 100644
--- a/devlib/target.py
+++ b/devlib/target.py
@@ -1077,6 +1077,12 @@
     def clear_logcat(self):
         adb_command(self.adb_name, 'logcat -c', timeout=30)
 
+    def adb_kill_server(self, timeout=30):
+        adb_command(self.adb_name, 'kill-server', timeout)
+
+    def adb_wait_for_device(self, timeout=30):
+        adb_command(self.adb_name, 'wait-for-device', timeout)
+
     def adb_reboot_bootloader(self, timeout=30):
         adb_command(self.adb_name, 'reboot-bootloader', timeout)