psh: ls-rootfs: add 'clear', 'reset' internal commands

Adds 'clear' and 'reset' commands to pshapp internal commands.

On large targets, these commands should be implemented by alias to
`tset` provided by system tools, while on small targets where psh
is used, pshapp controls the terminal (termios) directly, therefore
these commands have been implemented as internal (pshapp builtins).

JIRA: RTOS-561

psh/test-ls-rootfs.py

@@ -21,8 +21,8 @@
 
 
 def assert_ls_pshcmds(p, psh_cmds):
-    # history and exit symlinks shouldn't be present in bin
-    psh_cmds = set(psh_cmds) - {'history', 'exit'}
+    # history, exit, clear and reset symlinks shouldn't be present in bin
+    psh_cmds = set(psh_cmds) - {'history', 'exit', 'clear', 'reset'}
 
     psh_cmd_pattern = CONTROL_CODE
     psh_cmd_pattern += r'(?P<cmd>' + '|'.join(psh_cmds) + r')'