* Backport of GD32VF103 flash driver from https://github.com/riscv-mcu/riscv-openocd (GPL2 licensed)
Tested with a "Longan Nano" GD32VF103 dev board and seems to be working (flash can be read, erased and written).
* Modify src/flash/nor/gd32vf103.c to comply with Travis CI code style requirements.
* Modified README to include GD32VF103 in list of supported flash devices.