Added quoting to usb_producer key to preserve double quotes.

See arduino#1422.

Author: cmaglie

platform.txt

@@ -35,7 +35,7 @@ compiler.libsam.c.flags="-I{build.system.path}/libsam" "-I{build.system.path}/CM
 
 # USB Flags
 # ---------
-build.usb_flags=-DUSB_VID={build.vid} -DUSB_PID={build.pid} -DUSBCON -DUSB_MANUFACTURER={build.usb_manufacturer} '-DUSB_PRODUCT={build.usb_product}'
+build.usb_flags=-DUSB_VID={build.vid} -DUSB_PID={build.pid} -DUSBCON '-DUSB_MANUFACTURER={build.usb_manufacturer}' '-DUSB_PRODUCT={build.usb_product}'
 
 # Default usb manufacturer will be replaced at compile time using
 # numeric vendor ID if available or by board's specific value.