Program size check does not respect `board_upload.maximum_size` from `platformio.ini`

[einglis]

I'm building firmware for the Sonoff Basic device and it supports OTA updates. As a result it's important that the firmware isn't larger than half the total flash size.

Using board_build.ldscript = eagle.flash.1m64.ld I can influence the available space, as shown by post-build the size check:

Flash: [======    ]  58.7% (used 563013 bytes from 958448 bytes)

But:

• This value is not the same as in sonoff_basic.json: "upload": { ... "maximum_size": 1048576, and
• This value does not respect attempts to override it in platformio.ini: board_upload.maximum_size = 500000

What is happening is that in .platformio/platforms/espressif8266/builder/main.py, this action:

env.VerboseAction(
      lambda source, target, env: _update_max_upload_size(env),
      "Retrieving maximum program size $SOURCE"))

parses the .ld file and extracts the true flash size from this line:

  irom0_0_seg :                         org = 0x40201010, len = 0xe9ff0

but that all happens well after the user scripting stage. Without editing the .ld file, I can't see how to avoid this behaviour, and I don't want to do that anyway because:

• It's not maintainable
• It might have side effects I'm not aware of

One resolution would be for _update_max_upload_size to use the minimum of the current/scripted value (if non-zero) and the parsed value.