fixes infinite busy loop

[bmbove]

I'm not super well-versed in this codebase, but I've been struggling to upload and debug on an STM32L412K8

I've got it working with an instruction modification in the flash loader. Original code:

wait:
    ldrh r3, [r10]  ; load flash status register (upper) value into r3
    tst r3, #0x1    ; logical AND flash status register value and literal 0x01 (check FLASH_BSY flag for a 1)
    beq wait        ; if zero flag is set, jump to wait

when FLASH_BSY == 0, the tst instruction should set the zero flag. beq takes the branch if the 0 flag is set, so if flash is not busy, it continues to wait (incorrect behavior). changing beq to bne fixes this.

I'm hung up on this though because it seems like a really naive fix on my part. I'm willing to dig a little deeper if need be. This might help some folks with #607.

[bmbove]

actually, I'm going to investigate a little more.