* Migrate at_at/660m to use new ChibiOS early initialisation code, use generic F072 board.
* Migrate cannonkeys/satisfaction75 to use new ChibiOS early initialisation code, use generic F072 board.
* Migrate nebula12 to use new ChibiOS early initialisation code, use generic F072 board.
* Defer GENERIC_STM32_F072XB board files to ChibiOS.
* Create generic mcuconf.h file for GENERIC_STM32_F072XB.
* Create generic mcuconf.h file for STM32_F103_STM32DUINO.
* Create generic mcuconf.h file for GENERIC_STM32_F042X6.
* Allow boards to override EARLY_INIT_PERFORM_BOOTLOADER_JUMP.