Benchmark - FLASH and stack usage

[Crzyrndm]

This is intended for no_std environments, memory consumption is important.

Set up the standard slave example to cross compile to ARMv7m and check: at several opt levels (0, 1, "s", maybe more)

• the FLASH consumption
• max stack usage (https://blog.japaric.io/stack-analysis/)

opt="s" should probably be part of CI but that can come later