override OS=Linux Makefile variable to support building on macos

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

bossa only waited 1ms for the serial device instead calling
tcdrain().
Without this patch bossa is failing to flash a ATSAM3S4 8/10 times.
Patch has been submitted as PR#150 upstream.
https://github.com/shumatech/BOSSA/pull/150

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

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>