Merge pull request #761 from arduino/remove-prefix

remove trailing `-`: the IDE2 sets it automatically

Author: facchinm

platform.txt

@@ -214,7 +214,7 @@ tools.gdb.cmd.windows=arm-none-eabi-gdb.exe
 debug.executable={build.path}/{build.project_name}.elf
 debug.toolchain=gcc
 debug.toolchain.path={runtime.tools.arm-none-eabi-gcc-7-2017q4.path}/bin/
-debug.toolchain.prefix=arm-none-eabi-
+debug.toolchain.prefix=arm-none-eabi
 debug.server=openocd
 debug.server.openocd.path={runtime.tools.openocd-0.11.0-arduino2.path}/bin/openocd
 debug.server.openocd.scripts_dir={runtime.tools.openocd-0.11.0-arduino2.path}/share/openocd/scripts/