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.h b/board/technexion/twister/twister.h
index a2051c0..cff479c 100644
--- a/board/technexion/twister/twister.h
+++ b/board/technexion/twister/twister.h
@@ -38,6 +38,8 @@
 #define XR16L2751_UART1_BASE	0x21000000
 #define XR16L2751_UART2_BASE	0x23000000
 
+/* GPIO used to select between U-Boot and kernel */
+#define SPL_OS_BOOT_KEY	55
 
 /*
  * IEN  - Input Enable