Update MPU alignment checking logic

[bradjc]

The alignment logic for MPU on cortex-m in tockloader only checks if the app is aligned based on its entire size. However, the cortex-m mpu code includes more logic for using subregions:

https://github.com/tock/tock/blob/43429fe5866750302bce4758688d4c522cf719bd/arch/cortex-m/src/mpu.rs#L434-L502

Tockloader should replicate the same logic to provide a more accurate warning for apps which are not aligned to their entire size.