Linker error due to zigbee

[brentru]

@me-no-dev After running the absolute latest build.sh from master and then copying to Arduino (./tools/copy-to-arduino.sh), I get the following error when attempting to build:

xtensa-esp32s2-elf-g++: warning: {build.zigbee_mode}: linker input file unused because linking not done
xtensa-esp32s2-elf-g++: error: {build.zigbee_mode}: linker input file not found: No such file or directory

I am curious if it was caused by this PR, #146, and if I can remove it for the time being until the arduino esp32 BSP has zigbee as a managed component.

[me-no-dev]

fix is easy: just add the following two lines to platform.txt: https://github.com/espressif/arduino-esp32/pull/8881/files#diff-5c378d0844f0422d4d813eabe83f61ffce82014d8b8aa3e93ee35ce2ca14ca7bR89-R90

[brentru]

Works perfectly, thank you