Fixed to prevent error causing false positive

Returned string on missing file was non-empty, thus causing false detection of Pi. Fix tested and working on Mac OS Yosemite.

Author: Gadgetoid

makefile

@@ -20,7 +20,7 @@ VERSION			:=	$(shell echo $(shell grep -r VERSION= propelleride.pri \
 					| sed -e 's/ /./g')
 
 # if CPU (uname -m) equals...
-ifneq ($(shell cat /etc/os-release | grep "ID=raspbian"),"") # if Raspberry Pi
+ifeq ($(shell cat /etc/os-release | grep "ID=raspbian"),"ID=raspbian") # if Raspberry Pi
 	CPU := armhf
 else
 	ifeq ($(shell uname -m),i686)			# if i686