feat: add stm32h747 cortex-m7

feat: add stm32f407 device

feat: more robust ram data loading

The .data section was assumed to start
after .text. This condition is satisfied now,
but why rely on it? The load address can be exported
from the linker script with LOADADDR call.

fix: stm32f446 sram size

feat: add stm32f401 device

feat: add support for stm32f446

feat: add general template, Makefile, blink example

Initial commit