sdkconfig.defaults: use stub to flash, again

As we don't use the UART/JTAG detection mechanism anymore for rebooting
into bootrom serial, we can use the stub again, which makes things much
faster.

sdkconfig.defaults

 CONFIG_BOOTLOADER_LOG_LEVEL_WARN=y
 CONFIG_APP_EXCLUDE_PROJECT_VER_VAR=y
 CONFIG_APP_EXCLUDE_PROJECT_NAME_VAR=y
-CONFIG_ESPTOOLPY_NO_STUB=y
 CONFIG_ESPTOOLPY_FLASHMODE_QIO=y
 CONFIG_ESPTOOLPY_FLASHSIZE_16MB=y
 CONFIG_ESPTOOLPY_AFTER_NORESET=y