cmd: Introduce CMD_HELP

Add option to disable help command in size constrained systems to save some
space. There is also no need to have ifdefs around CMDLINE because all
commands depends on it.
And also mark cmd_help dependency in test_help.py.

Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/c17f825fb8a74e1d1912a3fd09a9a880c84a8bfd.1751286059.git.michal.simek@amd.com

test/py/tests/test_help.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+@pytest.mark.buildconfigspec('cmd_help')
 def test_help(ubman):
     """Test that the "help" command can be executed."""