nano_nora: conditionally enable debug button

If the `debug.executable` variable is null or unset, the Debug button
appears grayed out. The new IDE also takes current parameters into
account, allowing to have a conditional enable of the Debug button.

boards.txt

@@ -29755,6 +29755,7 @@ nano_nora.tools.esptool_py.program.pattern_args=--chip {build.mcu} --port "{seri
 nano_nora.tools.esptool_py.erase.pattern_args=--chip {build.mcu} --port "{serial.port}" --before default_reset --after hard_reset erase_flash
 
 nano_nora.programmer.default=esptool
+nano_nora.debug.executable=
 
 nano_nora.menu.PartitionScheme.default=With FAT partition (default)
 nano_nora.menu.PartitionScheme.spiffs=With SPIFFS partition (advanced)
@@ -29767,5 +29768,6 @@ nano_nora.menu.PinNumbers.byGPIONumber.build.disable_pin_remap=-DBOARD_USES_HW_G
 nano_nora.menu.USBMode.default=Normal mode (TinyUSB)
 nano_nora.menu.USBMode.hwcdc=Debug mode (Hardware CDC)
 nano_nora.menu.USBMode.hwcdc.build.usb_mode=1
+nano_nora.menu.USBMode.hwcdc.debug.executable={build.path}/{build.project_name}.elf
 
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