Makefile: Set default vendor SDK version to 2.1.0-18-g61248df (KRACK fix).

By the information provided by Espressif, this version contains fix
for the KRACK vulnerability.

More info: https://github.com/pfalcon/esp-open-sdk/issues/296

Makefile

@@ -10,7 +10,7 @@ TOOLCHAIN = $(TOP)/xtensa-lx106-elf
 
 # Vendor SDK version to install, see VENDOR_SDK_ZIP_* vars below
 # for supported versions.
-VENDOR_SDK = 2.0.0
+VENDOR_SDK = 2.1.0-18-g61248df
 
 .PHONY: crosstool-NG toolchain libhal libcirom sdk