cmd: pxe: CMD_PXE implies CMD_TFTPBOOT

cmd/pxe.c (CMD_PXE) calls do_get_tftp() (CMD_TFTPBOOT) therefore add
an "imply" to cmd/Kconfig.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 43f78a5..83c8281 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2099,6 +2099,7 @@
 config CMD_PXE
 	bool "pxe"
 	select PXE_UTILS
+	imply CMD_TFTPBOOT
 	help
 	  Boot image via network using PXE protocol