PIO: fix env definitions (again)

[mcspr]

2nd try for #2099
Resolve #2197

Move base envs to the same model as travis-{2_3_0,latest,git}:

• define env that sets platform, framework and build flags
• define 1m, 2m and 4m per each env
• every hw env sets extends = esp8266-{,latest,git}-SIZE-base instead of build_flags = ${common.build_flags_SIZE}

Additionally:

• remove esp8266-...-ota, Docs need adjustments
• remove every platform but 1.5.0 and 'latest', prepare to deprecate 1.5.0

Since we can't don't know how to force PIO to respect our wishes to not destroy build directory when our settings change or we add / remove files to project directory. Or, have any way of caching intermediate files that have the same source+build_flags but different env: https://community.platformio.org/t/build-cache-dir-will-not-share-object-files-between-envs/10011

So, instead, adjust build script to use another script that compiles env list into a compatible pio run calls that will re-use environment .o files cutting build time in half:
https://travis-ci.com/github/mcspr/espurna/builds/158421380 (~12-14min)
https://travis-ci.org/github/xoseperez/espurna/builds/631168730 (~27-28min)
But this forces us to use a very strict structure in .ini

TODO:

• I'll try to find another way of merging build_flags variables (or use the same mechanism as pio does + filter through uniq(). may even using pio config class directly, but we will depend on sort-of private api and we don't depend on a specific version)
• base-esp8266-1m-latest? esp8266-1m-latest-base?
• (?) instead of creating shell script, call pio run right in the python script

(
NOTE:
There is another approach that patches platformio internals at runtime:
https://github.com/esphome/esphome/blob/dev/esphome/platformio_api.py
Not sure I want to do that
)