Refactor MPS Makefile to support separate build directories

[spernsteiner]

This branch refactors the MPS src/Makefile to put all generated files (except the final binary) in a build directory and to use separate build directories for separate configurations. For example, this allows for the following:

make CONFIG=self_test
make CONFIG=no_self_test TARGET=aarch64

This produces separate rts.self_test and rts.no_self_test.aarch64 binaries. Object files and other intermediate files are placed in separate build.self_test/ and build.self_test.aarch64/ directories, so they don't conflict and there's no need to make clean between the two builds.

If CONFIG and TARGET aren't specified, it's still possible to configure the build by specifying options directly:

make SENSORS=NotSimulated SELF_TEST=Enabled \
  CC=aarch64-linux-gnu-gcc CXX=aarch64-linux-gnu-g++

This produces an unsuffixed rts binary and uses build/ for intermediate files.

Fixes #49

Checklist before requesting a review

• I have performed a self-review of my code
• My code matches the coding standards and I have ran the appropriate linters
• I included documentation updates for my code
• I extended the test suite and the tests run by the CI to cover my code
• I assigned a Milestone to this PR
• I assigned this PR to a Project
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[spernsteiner]

Turns out Ubuntu 22.04, which we use for CI, packages the correct verilator version for building MPS. This means we can install all the necessary dependencies via apt instead of using the HARDENS docker image, so we can get both the correct verilator and the aarch64 toolchain installed at the same time. I've updated the CI to build aarch64 binaries, which fixes #49.

[podhrmic]

LGTM!