Update boards.txt

added "build.usb_manufacturer" entry that overrides default macro in USBCore.cpp source file. this isn't really necessary since the source macro is the same as in "boards.txt", but it gives 3rd party board vendors direction in how to have their SAM3X board use their own manufacturer string during USB enumeration.

boards.txt

@@ -12,6 +12,7 @@ arduino_due_x_dbg.upload.wait_for_upload_port=false
 arduino_due_x_dbg.upload.native_usb=false
 arduino_due_x_dbg.build.mcu=cortex-m3
 arduino_due_x_dbg.build.f_cpu=84000000L
+arduino_due_x_dbg.build.usb_manufacturer="Arduino LLC"
 arduino_due_x_dbg.build.usb_product="Arduino Due"
 arduino_due_x_dbg.build.board=SAM_DUE
 arduino_due_x_dbg.build.core=arduino
@@ -35,6 +36,7 @@ arduino_due_x.upload.wait_for_upload_port=true
 arduino_due_x.upload.native_usb=true
 arduino_due_x.build.mcu=cortex-m3
 arduino_due_x.build.f_cpu=84000000L
+arduino_due_x.build.usb_manufacturer="Arduino LLC"
 arduino_due_x.build.usb_product="Arduino Due"
 arduino_due_x.build.board=SAM_DUE
 arduino_due_x.build.core=arduino