OMAP3: drop CONFIG_SPL_OS_BOOT_KEY and use local define

CONFIG_SPL_OS_BOOT_KEY is used only in board files. It is
not required to have a general CONFIG_ option. Rename it and
define it in board directory.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/board/technexion/twister/twister.c b/board/technexion/twister/twister.c
index c9eea9b..fa0ace0 100644
--- a/board/technexion/twister/twister.c
+++ b/board/technexion/twister/twister.c
@@ -165,10 +165,10 @@
 int spl_start_uboot(void)
 {
 	int val = 0;
-	if (!gpio_request(CONFIG_SPL_OS_BOOT_KEY, "U-Boot key")) {
-		gpio_direction_input(CONFIG_SPL_OS_BOOT_KEY);
-		val = gpio_get_value(CONFIG_SPL_OS_BOOT_KEY);
-		gpio_free(CONFIG_SPL_OS_BOOT_KEY);
+	if (!gpio_request(SPL_OS_BOOT_KEY, "U-Boot key")) {
+		gpio_direction_input(SPL_OS_BOOT_KEY);
+		val = gpio_get_value(SPL_OS_BOOT_KEY);
+		gpio_free(SPL_OS_BOOT_KEY);
 	}
 	return val;
 }