configs: m68k: Use default shell prompt

The current shell prompt '->' interferes with CI matching on 'bdinfo'
output. When CI test.py attempts to locate memory information in the
'bdinfo' output, it matches on '->' prefix which is identical to the
shell prefix. Switch the prompt to default '=>' one to avoid this
interference.

Suggested-by: Tom Rini <trini@konsulko.com> # found the CI oddity
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>