common: update: UPDATE_COMMON implies CMD_TFTPBOOT

common/update.c (UPDATE_COMMON) depends on tftp_timeout_ms which is
defined in cmd/tftp.c (CMD_TFTPBOOT). Therefore add the appropriate
imply statement to Kconfig.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
diff --git a/common/Kconfig b/common/Kconfig
index 83c81ed..968d4fb 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -934,6 +934,7 @@
 config UPDATE_COMMON
 	bool
 	select DFU_WRITE_ALT
+	imply CMD_TFTPBOOT
 
 config UPDATE_TFTP
 	bool "Auto-update using fitImage via TFTP"