Support for Espressif SDK (ESP32, etc)

Three environment variables must be set to use this variant:

   export ZEPHYR_GCC_VARIANT="espressif"
   export ESP_IDF_PATH=/path/to/esp-idf
   export ESPRESSIF_TOOLCHAIN_PATH=/path/to/xtensa-esp32-elf/

ESP-IDF is the SDK provided by Espressif.  It contains, among other things,
the HAL and header files for registers and ROM functions used by the Zephyr
port.  At this stage, with the exception of the HAL library, none of the
binary blobs provided by ESP-IDF are used.  This can be obtained directly
from Espressif, at <https://github.com/espressif/esp-idf

>.

Instructions on how to obtain the toolchain are detailed in the README for
ESP-IDF.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>