Makefile.include: remove repeated OS declaration

- OS := $(shell uname) is already declared in `Makefile.include`
  re-declaring it means it will be re-evalauted multiple times
  uselessly.

Makefile.include

@@ -103,7 +103,7 @@ CLEAN = $(filter clean, $(MAKECMDGOALS))
 include $(RIOTMAKE)/utils/variables.mk
 include $(RIOTMAKE)/utils/strings.mk
 
-# get host operating system
+# OS is always needed so use simple variable expansion so only evaluated once
 OS := $(shell uname)
 
 # set python path, e.g. for tests

boards/common/remote/Makefile.include

@@ -2,7 +2,6 @@
 export PROGRAMMER ?= cc2538-bsl
 
 ifeq ($(PROGRAMMER),cc2538-bsl)
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     PORT_BSL ?= $(PORT_LINUX)
   else ifeq ($(OS),Darwin)

boards/openmote-b/Makefile.include

@@ -6,7 +6,6 @@ PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbserial*)))
 export PROGRAMMER ?= cc2538-bsl
 
 ifeq ($(PROGRAMMER),cc2538-bsl)
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     PORT_BSL ?= $(PORT_LINUX)
   else ifeq ($(OS),Darwin)

makefiles/tools/bossa.inc.mk

@@ -5,7 +5,6 @@ FFLAGS  ?= -p $(PORT) -e -i -w -v -b -R $(FLASHFILE)
 # some arduino boards need to toggle the serial interface a little bit to get
 # them ready for flashing...
 ifneq (,$(BOSSA_ARDUINO_PREFLASH))
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     STTY_FLAG = -F
   else ifeq ($(OS),Darwin)

makefiles/tools/serial.inc.mk

@@ -1,5 +1,4 @@
 # set default port depending on operating system
-OS := $(shell uname)
 ifeq ($(OS),Linux)
   PORT ?= $(call ensure_value,$(PORT_LINUX),No port set)
 else ifeq ($(OS),Darwin)