Quality of life for development update

[zignig]

As the $FESTIVUS is upon us I have some spare cycles to actually work on fun stuff.

This pull request adds:

1. Specify the stack start in the bootloader as an environmental variable
2. Add a run command the to montool and defaulted to address to zero
3. Included my tinybxtest.py build script.

Projected works (in following pull requests):

1. Asyncify montool to include full serial terminal.
2. Work on SOCification.
3. Investigate adding optional CSR and WishBone interface for new amamranth-soc interface
   • https://github.com/jfng/amaranth-soc/tree/csr-reg-rfc

[zignig]

any opinion on this pull request ?

[zignig]

Obselete