Add MarlinSettings::validate()

Marlin/src/HAL/HAL_AVR/persistent_store_impl.cpp

Marlin/src/HAL/HAL_DUE/persistent_store_impl.cpp

Marlin/src/HAL/HAL_LPC1768/persistent_store_impl.cpp

Marlin/src/HAL/HAL_STM32F1/persistent_store_flash.cpp

Marlin/src/HAL/HAL_STM32F1/persistent_store_impl.cpp

Marlin/src/HAL/HAL_TEENSY35_36/persistent_store_impl.cpp

Marlin/src/HAL/persistent_store_api.h

Marlin/src/gcode/eeprom/M500-M504.cpp

Marlin/src/gcode/gcode.cpp

Marlin/src/gcode/gcode.h

Marlin/src/module/configuration_store.cpp

Marlin/src/module/configuration_store.h