Make clock source selectable

[maxgerhardt]

Currently the system_gd32f30x.c has a hardcoded clock source of HSE+PLL to 120MHz.

This should be changable via defines (in the Arduino IDE: via a menu selection) to be able to boot from

• IRC8M (internal 8MHz clock)
• IRC8M + PLL
• XTAL + PLL (current standard)
• external clock (XTAL in bypass mode)
• external clock (XTAL in bypass mode) + PLL