ARM: tegra: replace V_PROMPT define with kconfig

Commit 181bd9dc61d2 "kconfig: add config option for shell prompt" replaced
define V_PROMPT with Kconfig option SYS_PROMPT. This crossed with patches
adding Tegra T210 boards. Migrate the boards to the new scheme.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>
diff --git a/configs/p2371-2180_defconfig b/configs/p2371-2180_defconfig
index 83fde57..98448ee 100644
--- a/configs/p2371-2180_defconfig
+++ b/configs/p2371-2180_defconfig
@@ -15,3 +15,4 @@
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USE_PRIVATE_LIBGCC=y
+CONFIG_SYS_PROMPT="Tegra210 (P2371-2180) # "