Switch to SDK 2.0.0 by default.

Firmware built with SDK 2.0.0 can't be programmed to blank flash (will
lead to bootloop), so affected parties should be prepared to flash
esp_init_data_default.bin manually, or update their firmware to detect
blank flash and initialize default data automatically (what previous
versions of vendor SDK did themselves).

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 = 1.5.4
+VENDOR_SDK = 2.0.0
 
 .PHONY: crosstool-NG toolchain libhal libcirom sdk