tool: Fix 'idf.py bootloader' error if Secure Boot is enabled
tool: Fix 'idf.py bootloader' error if Secure Boot is enabled

If Secure Boot is enabled, there's no bootloader flasher arguments in the
project (to encourage careful manual flashing), so this command would
fail with a KeyError.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf