Fix RTC backup register indexing, add target-specific register and boot command

[twelho]

Continuation of my bug hunting and feature implementation round started in #41.

According to ST's RM0008 STM32F103xx reference manual section 6.4.5 table 17 the backup registers are 32 bits wide with only the lower 16 bits being used for storing values. In addition to this, the "first" register slot is entirely reserved and there is no backup register zero, meaning that BKP_DR1 starts from an offset of 0x04. This was mostly correctly reflected in backup.c, but the 2-multiplier for the register enum throws everything off by a lot, which results in the boot command check failing to work correctly when using anything but the first backup register. BKP0 has also been removed since as described by RM0008 indexing starts at one (BKP1 is now fully equivalent to the old BKP0).

I've also gone ahead and implemented support for specifying the backup register and boot command to use per-target. If not defined, options equivalent to the old behavior are used to not break existing applications.

[devanlai]

I probably should have better documented what I was doing with the backup registers here, sorry about that.

I originally started using the backup registers for DAP42 on an STM32F042 target (where the application instead uses the backup registers to check whether to trigger the ROM bootloader). When I later ported DAP42 to the STM32F103 and started work on dapboot, I wanted to make the backup register control for the STM32F103 look just like the backup register control for the STM32F042. On the STM32F0 series, the backup registers are 32-bits wide, so the backup implementation for the STM32F103 mimics that, using two physical 16-bit backup registers to implement one logical 32-bit backup register. Thus there are five logical backup registers 0-4 rather than the 10 physical backup registers 1-10 that actually exist on the STM32F103.

As far as I can tell, this already worked okay, or at least it worked for me when testing with logical backup registers 0, 1, and 4 (using physical registers 1 and 2, 3 and 4, and 9 and 10 respectively).

I like the idea of allowing for selecting other backup registers to use, though I had originally figured that anyone that wanted to customize the bootloader backup registers would also probably want to change it to use a single physical backup register instead of two, anyways.

[twelho]

The STM32F103xx series actually has 42 backup registers, but that would quite severely bloat the BackupRegister enum if I would write them all out. I went up to 10 since that is what BTTSKRMINIE3V2 in #44 requires, and I think if someone needs more they can open a new PR to add them later.

Since the bootloader targets the STM32F103xx series MCUs it only makes sense to use their register naming IMO. If both the bootloader and the application refer to a different memory location with e.g. BKP10 it will only cause needless debugging of why the MCU didn't stay in the bootloader when the application told it to (as happened in my case).

Do you want me to change stm32f103/backup.c to use a single physical backup register instead of two? The bootloader using two registers is a bit unexpected for many applications targeting the STM32F103xx series. Moving to a single backup register should technically not break backwards compatibility either, the second register written by the application will simply not be considered by the bootloader anymore, but functionality will (in most cases) remain the same.

[twelho]

30e54d0 now contains the change to only use one (the first) backup register. LMK if this is acceptable or if it's better to keep the old behavior.