picosam9g45: adopt CONFIG_SYS_PROMPT

Commit 181bd9dc61d2da88b78f1c1138a685dae39354d6 introduced Kconfig selection
for SYS_PROMPT. When applying the new picosam9g45 board this change slipped
through, adopt it.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
2 files changed