esp32: Update to latest ESP IDF using sdkconfig and new ldgen procedure.

Configuration for the build is now specified using sdkconfig rather than
sdkconfig.h, which allows for much easier configuration with defaults from
the ESP IDF automatically applied.  sdkconfig.h is generated using the new
ESP IDF kconfig_new tool written in Python.  Custom configuration for a
particular ESP32 board can be specified via the make variable SDKCONFIG.

The esp32.common.ld file is also now generated using the standard ESP IDF
ldgen.py tool.