py: Add NLR support for xtensa CPU.
esp8266: New port of Micro Python to ESP8266 wifi module.
esp8266: Allow to override serial port.
esp8266: Provide sensible default ESP_SDK; deploy 2 the images together.
esp8266: Change bignum from mpz to longlong; move some rodata to iram.

Some rodata items can go in iram/irom segment, but not others.  With
this patch ESP now has 24256 bytes of heap ram.  It passes 228 out of
248 tests from tests/basics directory.