Several small fixes for broader hardware support

[pierstitus]

As part of using this bootloader for my project (www.striso.org) I ran into some issues while porting to different hardware.

• added hasScreen checks (I don't use a screen)
• build before flash-ocd target
• init BOARD_FORCE_BL pin with pullup/pulldown
• add stm32f4 discovery board for easier testing/development
• add GPIO D-K, only port A-C where initialized

[mmoskal]

Looks great, thank you!