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>
diff --git a/configs/picosam9g45_defconfig b/configs/picosam9g45_defconfig
index 4128193..ccca1ca 100644
--- a/configs/picosam9g45_defconfig
+++ b/configs/picosam9g45_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_PICOSAM9G45=y
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,SYS_USE_MMC"
+CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set