devlib: Remove sudo prompt

Since the prompt is added to stdout, remove the one-space-prompt that
currently corrupts stdout when a command is ran with sudo.

That non-empty prompt was added as Windows Subsystem for Linux version 1
(WSL1) has a broken sudo implementation that chokes on an empty prompt.
Considering this is not a platform that is normally suported by devlib,
we re-introduce that empty prompt.

devlib/host.py

@@ -112,6 +112,8 @@ class LocalConnection(ConnectionBase):
             if self.unrooted:
                 raise TargetStableError('unrooted')
             password = self._get_password()
+            # Empty prompt with -p '' to avoid adding a leading space to the
+            # output.
             command = "echo {} | sudo -k -p '' -S -- sh -c {}".format(quote(password), quote(command))
         ignore = None if check_exit_code else 'all'
         try:
@@ -136,8 +138,8 @@ class LocalConnection(ConnectionBase):
             if self.unrooted:
                 raise TargetStableError('unrooted')
             password = self._get_password()
-            # The sudo prompt will add a space on stderr, but we cannot filter
-            # it out here
+            # Empty prompt with -p '' to avoid adding a leading space to the
+            # output.
             command = "echo {} | sudo -k -p '' -S -- sh -c {}".format(quote(password), quote(command))
 
         # Make sure to get a new PGID so PopenBackgroundCommand() can kill

devlib/utils/ssh.py

@@ -61,6 +61,7 @@ from devlib.utils.types import boolean
 from devlib.connection import ConnectionBase, ParamikoBackgroundCommand, SSHTransferHandle
 
 
+# Empty prompt with -p '' to avoid adding a leading space to the output.
 DEFAULT_SSH_SUDO_COMMAND = "sudo -k -p '' -S -- sh -c {}"