No way to add build flags

[Kernald]

Before PlatformIO 4 (I'm not 100% sure it's the actual cause, but it's the likely culprit), passing two build_flags options to the same section of the platformio.ini kind of worked (I guess the last one was overriding the previous ones silently, which wasn't great either), and as a result, doing something like this was an option:

platformio_project(
    name = "test",
    src = "main.cc",
    board = "nodemcuv2",
    environment_kwargs = {
        "build_flags": "-DMQTT_SOCKET_TIMEOUT=60 -DMQTT_KEEPALIVE=60 -DMQTT_MAX_PACKET_SIZE=256 -DSERIAL_SPEED=9600",
    },
    framework = "arduino",
    platform = "espressif8266",
    deps = [
        ...
    ],
)

PlatformIO now (rightfully) complains about build_flags being defined twice - it's indeed defined in the template. It would be nice to have an extension point to those flags.