Use linker symbols for mtvec instead of hardcoded addresses

[aq1018]

I have a bootloader running before the real application. This means the application code starts at an offset instead of the default 0x00 location. This PR allows the proper resolution of the vector table for chain loaded applications.

Changes:

• Use linker script symbols: _start and _highcode_vma_start as the mtvec address.
• Added ASSERT linker script assertions to ensure vector tables align to 1024 bytes.

[aq1018]

p.s. I tested this locally and it works for both default use cases and chain loaded apps. I also tested the linker script asserts by messing with memory.x FLASH and RAM addresses and the assertions stops compilation properly, the error message might be a bit vague, and feel free to ask me to modify if it's not sufficient.

I also noticed that 0.6.1 code has not been synced to main branch yet. I filed a separate issue (#18). Feel free to let me know if I need rebasing if / when you add 0.6.1 change sets.

Thank you for your time!