platform: fix default debug prefix

"debug.toolchain.prefix" must end with a dash, since only the tool name is
appended to this string.

The reason this is not a major issue is that the "debug_custom.json" file
(copied in the sketch directory when debugging is enabled) forces its own
prefix. And to make things more interesting, the "toolchainPrefix" entry
in that file should _not_ end with a dash.

Author: pillo79

platform.txt

@@ -224,7 +224,7 @@ pluggable_monitor.required.serial=builtin:serial-monitor
 debug.executable={build.path}/{build.project_name}.elf
 debug.toolchain=gcc
 debug.toolchain.path={tools.{build.tarch}-esp-elf-gdb.path}/bin/
-debug.toolchain.prefix={build.tarch}-{build.target}-elf
+debug.toolchain.prefix={build.tarch}-{build.target}-elf-
 debug.server=openocd
 debug.server.openocd.script=debug.cfg