[dv,ralgen] revert `ralgen.py` to use relative file paths
[dv,ralgen] revert `ralgen.py` to use relative file paths

`dvsim.py` supports moving the scratch working area to another location
other than inside the main repo. This is achieved by setting the
SCRATCH_ROOT environment variable or the --scratch-root flag on the
invocation of `dvsim.py`.

`dvsim.py` itself invokes hw and sw builds by calling the makefile
`hw/dv/tools/dvsim/sim.mk`. In the gen_svn_filelist target the following
happens:
  - cd ${build_dir} – at this point build_dir is set to SCRATCH_ROOT
  - FuseSoC targets are built
  - ralgen.py is called
  - ralgen.py now uses git rev-parse to find REPO_TOP, but this does NOT
    work when SCRATCH_ROOT is set to outside of the git repo.

This commit reverts #13071 which made the change to locate the
`topgen.py` via a `git rev-parse ...` command.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

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