fix Travis error
Adding files for STM32F1 HAL based on libmaple/stm32duino core. Current persistent_store uses cardreader changes to be sent in separate commit, but could be changed to use i2c eeprom.
Replace ARDUINO_ARCH_AVR with __AVR__ to better detect architecture for non-Arduino dev environments. Resolves compile failure in PIO for 8-bit Teensduino targets More info: https://forum.pjrc.com/threads/33234-Using-Teensyduino-Selecting-Teensy-3-2-3-1-board-has-incorrect-platform-define http://www.atmel.com/webdoc/avrlibcreferencemanual/using_tools_1using_avr_gcc_mach_opt.html
Implement AVR Platform