remove .cpp from the resulting suffix, since .ipp already implies it is c++
update comments once again

fixes #2479

Generate resulting .cpp in the $BUILD_DIR, no need for atexit
Register middleware only once, there's no need to manually walk the
project directory since fnmatch.fnmatch(...) supports globs

Provide a SCons Action instead of simple function, so there's a way to
generate things on demand via command line
```
$ pio run -e $env -t .pio/build/$env/espurna_single_source/src/main.cpp
```

- just pass `-include ${cfg}` to the compiler instead of a
  temporary custom.h. also do `-DDEVICE=...` and `-DMANUFACTURER=...`
- rework test_build to print using `logging` module
- report leftover configurations, after the failed one exits
- prettify the output and highlight relevant info instead of the whole lines

- provide a queue for the outgoing messages, don't depend on the MQTT
  callback implementation details and always postpone the actual transfer.
- generate user input parsers using re2c, adds special PlatformIO build handlers
- add terminal command to show GPIO info and send a simple payload
- rework button<->action presets into value<->terminal command presets and custom ones
  through the use of `irCmd<VALUE>` settings, where the VALUE is taken from the simple payload
  (yet, it does not understand repeated codes, but that's something to add later *or* use rpnlib)
- add tx message integration for the relay module via `irRelayOn#` and `irRelayOff#`
- rework simple payload to include 'repeats' value for the 'IRsend::send()'
  also adds internal 'series', 'delay' and includes full 64bit 'value'
  (as uppercase HEX, instead of decimal)
- rework raw payload to use 'series' instead of 'repeats', and provide a
  clear distinction between the usec time and the options of the message
  by moving the required options to the front and separating them using ':'
  (just like the simple variant)
- make RX and TX pin a runtime setting, make RX and TX support a build flag
- small test framework to check whether internal string encode<->decode works

Also updates the hexEncode & hexDecode implementations to use
'iterators' instead of just pointer + index.

Multiple ways to specify version string through environment variables:
- `ESPURNA_BUILD_FULL_VERSION` to set full version string
By default it is empty, and the version is combined using the values specified below
- `ESPURNA_BUILD_VERSION` to modify the first part of the version string (1.15.0-dev)
By default, uses espurna/config/version.h APP_VERSION value
- `ESPURNA_BUILD_REVISION` to specify revision part of the version string
For example 12345678, which is expanded as either .git12345678 or -git12345678
(depending on whether the version string contains hyphen)
By default, use git to retrieve the first 8 characters of the current HEAD SHA value
- `ESPURNA_BUILD_VERSION_SUFFIX` to specify build metadata part of the string
For example nightly20210607, which is added to the full version as 1.15.0-dev+nightly20210607
Empty by defauld

Adds -t build-and-copy which uses the values above to copy firmware.bin, configurable with:
- `ESPURNA_BUILD_NAME` to set the suffix of the filename.
By default, uses the $PIOENV (aka the string after the env: in the .ini file)
- `ESPURNA_BUILD_DESTINATION` to specify where to copy the .bin files
By default, uses $PROJECT_DIR

Resulting file is stored at:
${ESPURNA_BUILD_DESTINATION}/${ESPURNA_BUILD_FULL_VERSION}/espurna-${ESPURNA_BUILD_FULL_VERSION}-${ESPURNA_BUILD_NAME}.bin

In addition, modify generate_release_sh.py to use the new environment variables.