override OS=Linux Makefile variable to support building on macos

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>

Fixes: #17287 ("bossa: recompile with -fPIC")

Signed-off-by: Nick Hainke <vincent@systemli.org>

Based on the work of Hirokazu MORIKAWA (nxhack):
https://github.com/nxhack/openwrt-arduino-packages/tree/master/bossa

BOSSA is a flash programming utility for Atmel's SAM family of
flash-based ARM microcontrollers. The motivation behind BOSSA is to
create a simple, easy-to-use, open source utility to replace Atmel's
SAM-BA software.

Signed-off-by: Nick Hainke <vincent@systemli.org>