ARM: tegra: convert CFG_TEGRA_BOARD_STRING into Kconfig option

Convert CFG_TEGRA_BOARD_STRING into Kconfig option and move it into device
board Kconfig.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
diff --git a/board/avionic-design/plutux/Kconfig b/board/avionic-design/plutux/Kconfig
index 09a3ac9..57324dc 100644
--- a/board/avionic-design/plutux/Kconfig
+++ b/board/avionic-design/plutux/Kconfig
@@ -9,4 +9,8 @@
 config SYS_CONFIG_NAME
 	default "plutux"
 
+config TEGRA_BOARD_STRING
+	string "Default Tegra board name"
+	default "Avionic Design Plutux"
+
 endif