Fix PlatformIO linking for variant-specific overrides

[maxgerhardt]

Fixes the issue discussed in #66 (comment). The issue description and fix are basically the same as in espressif/arduino-esp32#6809.

Test case

[env:rpipicow]
platform = https://github.com/maxgerhardt/platform-raspberrypi.git
framework = arduino
board = rpipicow
board_build.core = earlephilhower
board_build.filesystem_size = 0.5m

Before fix: initVariant is an empty function, returns immediately (the _weak default)

> C:\Users\Max\.platformio\packages\toolchain-rp2040-earlephilhower\bin\arm-none-eabi-objdump.exe -d .\.pio\build\rpipicow\firmware.elf  | grep -A5 initVariant
100008a4 <initVariant>:
100008a4:       4770            bx      lr

After fix: initVariant for the RPiPico W calls into the WiFi init function

> C:\Users\Max\.platformio\packages\toolchain-rp2040-earlephilhower\bin\arm-none-eabi-objdump.exe -d .\.pio\build\rpipicow\firmware.elf  | grep -A5 initVariant
1000032c <initVariant>:
1000032c:       b510            push    {r4, lr}
1000032e:       f000 fc85       bl      10000c3c <cyw43_arch_init>
10000332:       bd10            pop     {r4, pc}