sandbox: Set CONFIG_SYS_CPU

Give a valid value here as well to allow things which want
CONFIG_ENV_VARS_UBOOT_CONFIG to build

Signed-off-by: Tom Rini <trini@konsulko.com>

arch/sandbox/Kconfig

@@ -7,6 +7,9 @@ config SYS_ARCH
 config SYS_BOARD
 	default "sandbox"
 
+config SYS_CPU
+	default "sandbox"
+
 config SYS_CONFIG_NAME
 	default "sandbox"