kunit: tool: cosmetic: don't specify duplicate kernel cmdline options

Context:
When using a non-UML arch, kunit.py will boot the test kernel with
options like these by default (this is x86_64):
> mem=1G console=tty kunit_shutdown=halt console=ttyS0 kunit_shutdown=reboot

The first three options are added unconditionally but are only intended
for UML.

1. 'mem=1G' is redundant with the '-m 1024' that we hard-code into the
   qemu commandline.

2. We specify a 'console' for all tools/testing/kunit/qemu_configs/*.py
   already, so 'console=tty' gets overwritten.

3. For QEMU, we need to use 'reboot', and for UML we need to use 'halt'.
   If you switch them, kunit.py will hang until the --timeout expires.

This patch:
Having these duplicate options is a bit noisy.
Switch so we only add UML-specific options for UML.

I.e. we now get
UML: 'mem=1G console=tty kunit_shutdown=halt' (unchanged)
x86_64: 'console=ttyS0 kunit_shutdown=reboot'

Side effect: you can't overwrite these options on UML w/ --kernel_arg.
But you already couldn't for QEMU (console, kunit_shutdown), and why
would you want to?

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>